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<div class="yui-g" id="ref-models-instances">
<div class="section" id="s-model-instance-reference">
<span id="model-instance-reference"></span><h1>Model instance reference<a class="headerlink" href="#model-instance-reference" title="Permalink to this headline">¶</a></h1>
<p>This document describes the details of the <code class="docutils literal notranslate"><span class="pre">Model</span></code> API. It builds on the
material presented in the <a class="reference internal" href="../../topics/db/models.html"><span class="doc">model</span></a> and <a class="reference internal" href="../../topics/db/queries.html"><span class="doc">database
query</span></a> guides, so you’ll probably want to read and
understand those documents before reading this one.</p>
<p>Throughout this reference we’ll use the <a class="reference internal" href="../../topics/db/queries.html#queryset-model-example"><span class="std std-ref">example Weblog models</span></a> presented in the <a class="reference internal" href="../../topics/db/queries.html"><span class="doc">database query guide</span></a>.</p>
<div class="section" id="s-creating-objects">
<span id="creating-objects"></span><h2>Creating objects<a class="headerlink" href="#creating-objects" title="Permalink to this headline">¶</a></h2>
<p>To create a new instance of a model, just instantiate it like any other Python
class:</p>
<dl class="class">
<dt id="django.db.models.Model">
<em class="property">class </em><code class="descname">Model</code>(<em>**kwargs</em>)<a class="reference internal" href="../../_modules/django/db/models/base.html#Model"><span class="viewcode-link">[source]</span></a><a class="headerlink" href="#django.db.models.Model" title="Permalink to this definition">¶</a></dt>
<dd></dd></dl>
<p>The keyword arguments are simply the names of the fields you’ve defined on your
model. Note that instantiating a model in no way touches your database; for
that, you need to <a class="reference internal" href="#django.db.models.Model.save" title="django.db.models.Model.save"><code class="xref py py-meth docutils literal notranslate"><span class="pre">save()</span></code></a>.</p>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p>You may be tempted to customize the model by overriding the <code class="docutils literal notranslate"><span class="pre">__init__</span></code>
method. If you do so, however, take care not to change the calling
signature as any change may prevent the model instance from being saved.
Rather than overriding <code class="docutils literal notranslate"><span class="pre">__init__</span></code>, try using one of these approaches:</p>
<ol class="last arabic">
<li><p class="first">Add a classmethod on the model class:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">django.db</span> <span class="k">import</span> <span class="n">models</span>
<span class="k">class</span> <span class="nc">Book</span><span class="p">(</span><span class="n">models</span><span class="o">.</span><span class="n">Model</span><span class="p">):</span>
<span class="n">title</span> <span class="o">=</span> <span class="n">models</span><span class="o">.</span><span class="n">CharField</span><span class="p">(</span><span class="n">max_length</span><span class="o">=</span><span class="mi">100</span><span class="p">)</span>
<span class="nd">@classmethod</span>
<span class="k">def</span> <span class="nf">create</span><span class="p">(</span><span class="bp">cls</span><span class="p">,</span> <span class="n">title</span><span class="p">):</span>
<span class="n">book</span> <span class="o">=</span> <span class="bp">cls</span><span class="p">(</span><span class="n">title</span><span class="o">=</span><span class="n">title</span><span class="p">)</span>
<span class="c1"># do something with the book</span>
<span class="k">return</span> <span class="n">book</span>
<span class="n">book</span> <span class="o">=</span> <span class="n">Book</span><span class="o">.</span><span class="n">create</span><span class="p">(</span><span class="s2">"Pride and Prejudice"</span><span class="p">)</span>
</pre></div>
</div>
</li>
<li><p class="first">Add a method on a custom manager (usually preferred):</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">BookManager</span><span class="p">(</span><span class="n">models</span><span class="o">.</span><span class="n">Manager</span><span class="p">):</span>
<span class="k">def</span> <span class="nf">create_book</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">title</span><span class="p">):</span>
<span class="n">book</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">create</span><span class="p">(</span><span class="n">title</span><span class="o">=</span><span class="n">title</span><span class="p">)</span>
<span class="c1"># do something with the book</span>
<span class="k">return</span> <span class="n">book</span>
<span class="k">class</span> <span class="nc">Book</span><span class="p">(</span><span class="n">models</span><span class="o">.</span><span class="n">Model</span><span class="p">):</span>
<span class="n">title</span> <span class="o">=</span> <span class="n">models</span><span class="o">.</span><span class="n">CharField</span><span class="p">(</span><span class="n">max_length</span><span class="o">=</span><span class="mi">100</span><span class="p">)</span>
<span class="n">objects</span> <span class="o">=</span> <span class="n">BookManager</span><span class="p">()</span>
<span class="n">book</span> <span class="o">=</span> <span class="n">Book</span><span class="o">.</span><span class="n">objects</span><span class="o">.</span><span class="n">create_book</span><span class="p">(</span><span class="s2">"Pride and Prejudice"</span><span class="p">)</span>
</pre></div>
</div>
</li>
</ol>
</div>
<div class="section" id="s-customizing-model-loading">
<span id="customizing-model-loading"></span><h3>Customizing model loading<a class="headerlink" href="#customizing-model-loading" title="Permalink to this headline">¶</a></h3>
<dl class="classmethod">
<dt id="django.db.models.Model.from_db">
<em class="property">classmethod </em><code class="descclassname">Model.</code><code class="descname">from_db</code>(<em>db</em>, <em>field_names</em>, <em>values</em>)<a class="reference internal" href="../../_modules/django/db/models/base.html#Model.from_db"><span class="viewcode-link">[source]</span></a><a class="headerlink" href="#django.db.models.Model.from_db" title="Permalink to this definition">¶</a></dt>
<dd></dd></dl>
<p>The <code class="docutils literal notranslate"><span class="pre">from_db()</span></code> method can be used to customize model instance creation
when loading from the database.</p>
<p>The <code class="docutils literal notranslate"><span class="pre">db</span></code> argument contains the database alias for the database the model
is loaded from, <code class="docutils literal notranslate"><span class="pre">field_names</span></code> contains the names of all loaded fields, and
<code class="docutils literal notranslate"><span class="pre">values</span></code> contains the loaded values for each field in <code class="docutils literal notranslate"><span class="pre">field_names</span></code>. The
<code class="docutils literal notranslate"><span class="pre">field_names</span></code> are in the same order as the <code class="docutils literal notranslate"><span class="pre">values</span></code>. If all of the model’s
fields are present, then <code class="docutils literal notranslate"><span class="pre">values</span></code> are guaranteed to be in the order
<code class="docutils literal notranslate"><span class="pre">__init__()</span></code> expects them. That is, the instance can be created by
<code class="docutils literal notranslate"><span class="pre">cls(*values)</span></code>. If any fields are deferred, they won’t appear in
<code class="docutils literal notranslate"><span class="pre">field_names</span></code>. In that case, assign a value of <code class="docutils literal notranslate"><span class="pre">django.db.models.DEFERRED</span></code>
to each of the missing fields.</p>
<p>In addition to creating the new model, the <code class="docutils literal notranslate"><span class="pre">from_db()</span></code> method must set the
<code class="docutils literal notranslate"><span class="pre">adding</span></code> and <code class="docutils literal notranslate"><span class="pre">db</span></code> flags in the new instance’s <code class="docutils literal notranslate"><span class="pre">_state</span></code> attribute.</p>
<p>Below is an example showing how to record the initial values of fields that
are loaded from the database:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">django.db.models</span> <span class="k">import</span> <span class="n">DEFERRED</span>
<span class="nd">@classmethod</span>
<span class="k">def</span> <span class="nf">from_db</span><span class="p">(</span><span class="bp">cls</span><span class="p">,</span> <span class="n">db</span><span class="p">,</span> <span class="n">field_names</span><span class="p">,</span> <span class="n">values</span><span class="p">):</span>
<span class="c1"># Default implementation of from_db() (subject to change and could</span>
<span class="c1"># be replaced with super()).</span>
<span class="k">if</span> <span class="nb">len</span><span class="p">(</span><span class="n">values</span><span class="p">)</span> <span class="o">!=</span> <span class="nb">len</span><span class="p">(</span><span class="bp">cls</span><span class="o">.</span><span class="n">_meta</span><span class="o">.</span><span class="n">concrete_fields</span><span class="p">):</span>
<span class="n">values</span> <span class="o">=</span> <span class="nb">list</span><span class="p">(</span><span class="n">values</span><span class="p">)</span>
<span class="n">values</span><span class="o">.</span><span class="n">reverse</span><span class="p">()</span>
<span class="n">values</span> <span class="o">=</span> <span class="p">[</span>
<span class="n">values</span><span class="o">.</span><span class="n">pop</span><span class="p">()</span> <span class="k">if</span> <span class="n">f</span><span class="o">.</span><span class="n">attname</span> <span class="ow">in</span> <span class="n">field_names</span> <span class="k">else</span> <span class="n">DEFERRED</span>
<span class="k">for</span> <span class="n">f</span> <span class="ow">in</span> <span class="bp">cls</span><span class="o">.</span><span class="n">_meta</span><span class="o">.</span><span class="n">concrete_fields</span>
<span class="p">]</span>
<span class="n">instance</span> <span class="o">=</span> <span class="bp">cls</span><span class="p">(</span><span class="o">*</span><span class="n">values</span><span class="p">)</span>
<span class="n">instance</span><span class="o">.</span><span class="n">_state</span><span class="o">.</span><span class="n">adding</span> <span class="o">=</span> <span class="kc">False</span>
<span class="n">instance</span><span class="o">.</span><span class="n">_state</span><span class="o">.</span><span class="n">db</span> <span class="o">=</span> <span class="n">db</span>
<span class="c1"># customization to store the original field values on the instance</span>
<span class="n">instance</span><span class="o">.</span><span class="n">_loaded_values</span> <span class="o">=</span> <span class="nb">dict</span><span class="p">(</span><span class="nb">zip</span><span class="p">(</span><span class="n">field_names</span><span class="p">,</span> <span class="n">values</span><span class="p">))</span>
<span class="k">return</span> <span class="n">instance</span>
<span class="k">def</span> <span class="nf">save</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="o">*</span><span class="n">args</span><span class="p">,</span> <span class="o">**</span><span class="n">kwargs</span><span class="p">):</span>
<span class="c1"># Check how the current values differ from ._loaded_values. For example,</span>
<span class="c1"># prevent changing the creator_id of the model. (This example doesn't</span>
<span class="c1"># support cases where 'creator_id' is deferred).</span>
<span class="k">if</span> <span class="ow">not</span> <span class="bp">self</span><span class="o">.</span><span class="n">_state</span><span class="o">.</span><span class="n">adding</span> <span class="ow">and</span> <span class="p">(</span>
<span class="bp">self</span><span class="o">.</span><span class="n">creator_id</span> <span class="o">!=</span> <span class="bp">self</span><span class="o">.</span><span class="n">_loaded_values</span><span class="p">[</span><span class="s1">'creator_id'</span><span class="p">]):</span>
<span class="k">raise</span> <span class="ne">ValueError</span><span class="p">(</span><span class="s2">"Updating the value of creator isn't allowed"</span><span class="p">)</span>
<span class="nb">super</span><span class="p">(</span><span class="o">...</span><span class="p">)</span><span class="o">.</span><span class="n">save</span><span class="p">(</span><span class="o">*</span><span class="n">args</span><span class="p">,</span> <span class="o">**</span><span class="n">kwargs</span><span class="p">)</span>
</pre></div>
</div>
<p>The example above shows a full <code class="docutils literal notranslate"><span class="pre">from_db()</span></code> implementation to clarify how that
is done. In this case it would of course be possible to just use <code class="docutils literal notranslate"><span class="pre">super()</span></code> call
in the <code class="docutils literal notranslate"><span class="pre">from_db()</span></code> method.</p>
<div class="versionchanged">
<span class="title">Changed in Django 1.10:</span> <p>In older versions, you could check if all fields were loaded by consulting
<code class="docutils literal notranslate"><span class="pre">cls._deferred</span></code>. This attribute is removed and
<code class="docutils literal notranslate"><span class="pre">django.db.models.DEFERRED</span></code> is new.</p>
</div>
</div>
</div>
<div class="section" id="s-refreshing-objects-from-database">
<span id="refreshing-objects-from-database"></span><h2>Refreshing objects from database<a class="headerlink" href="#refreshing-objects-from-database" title="Permalink to this headline">¶</a></h2>
<p>If you delete a field from a model instance, accessing it again reloads the
value from the database:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">obj</span> <span class="o">=</span> <span class="n">MyModel</span><span class="o">.</span><span class="n">objects</span><span class="o">.</span><span class="n">first</span><span class="p">()</span>
<span class="gp">>>> </span><span class="k">del</span> <span class="n">obj</span><span class="o">.</span><span class="n">field</span>
<span class="gp">>>> </span><span class="n">obj</span><span class="o">.</span><span class="n">field</span> <span class="c1"># Loads the field from the database</span>
</pre></div>
</div>
<div class="versionchanged">
<span class="title">Changed in Django 1.10:</span> <p>In older versions, accessing a deleted field raised <code class="docutils literal notranslate"><span class="pre">AttributeError</span></code>
instead of reloading it.</p>
</div>
<dl class="method">
<dt id="django.db.models.Model.refresh_from_db">
<code class="descclassname">Model.</code><code class="descname">refresh_from_db</code>(<em>using=None</em>, <em>fields=None</em>)<a class="reference internal" href="../../_modules/django/db/models/base.html#Model.refresh_from_db"><span class="viewcode-link">[source]</span></a><a class="headerlink" href="#django.db.models.Model.refresh_from_db" title="Permalink to this definition">¶</a></dt>
<dd></dd></dl>
<p>If you need to reload a model’s values from the database, you can use the
<code class="docutils literal notranslate"><span class="pre">refresh_from_db()</span></code> method. When this method is called without arguments the
following is done:</p>
<ol class="arabic simple">
<li>All non-deferred fields of the model are updated to the values currently
present in the database.</li>
<li>The previously loaded related instances for which the relation’s value is no
longer valid are removed from the reloaded instance. For example, if you have
a foreign key from the reloaded instance to another model with name
<code class="docutils literal notranslate"><span class="pre">Author</span></code>, then if <code class="docutils literal notranslate"><span class="pre">obj.author_id</span> <span class="pre">!=</span> <span class="pre">obj.author.id</span></code>, <code class="docutils literal notranslate"><span class="pre">obj.author</span></code> will
be thrown away, and when next accessed it will be reloaded with the value of
<code class="docutils literal notranslate"><span class="pre">obj.author_id</span></code>.</li>
</ol>
<p>Only fields of the model are reloaded from the database. Other
database-dependent values such as annotations aren’t reloaded. Any
<a class="reference internal" href="../utils.html#django.utils.functional.cached_property" title="django.utils.functional.cached_property"><code class="xref py py-func docutils literal notranslate"><span class="pre">@cached_property</span></code></a> attributes
aren’t cleared either.</p>
<p>The reloading happens from the database the instance was loaded from, or from
the default database if the instance wasn’t loaded from the database. The
<code class="docutils literal notranslate"><span class="pre">using</span></code> argument can be used to force the database used for reloading.</p>
<p>It is possible to force the set of fields to be loaded by using the <code class="docutils literal notranslate"><span class="pre">fields</span></code>
argument.</p>
<p>For example, to test that an <code class="docutils literal notranslate"><span class="pre">update()</span></code> call resulted in the expected
update, you could write a test similar to this:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">test_update_result</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="n">obj</span> <span class="o">=</span> <span class="n">MyModel</span><span class="o">.</span><span class="n">objects</span><span class="o">.</span><span class="n">create</span><span class="p">(</span><span class="n">val</span><span class="o">=</span><span class="mi">1</span><span class="p">)</span>
<span class="n">MyModel</span><span class="o">.</span><span class="n">objects</span><span class="o">.</span><span class="n">filter</span><span class="p">(</span><span class="n">pk</span><span class="o">=</span><span class="n">obj</span><span class="o">.</span><span class="n">pk</span><span class="p">)</span><span class="o">.</span><span class="n">update</span><span class="p">(</span><span class="n">val</span><span class="o">=</span><span class="n">F</span><span class="p">(</span><span class="s1">'val'</span><span class="p">)</span> <span class="o">+</span> <span class="mi">1</span><span class="p">)</span>
<span class="c1"># At this point obj.val is still 1, but the value in the database</span>
<span class="c1"># was updated to 2. The object's updated value needs to be reloaded</span>
<span class="c1"># from the database.</span>
<span class="n">obj</span><span class="o">.</span><span class="n">refresh_from_db</span><span class="p">()</span>
<span class="bp">self</span><span class="o">.</span><span class="n">assertEqual</span><span class="p">(</span><span class="n">obj</span><span class="o">.</span><span class="n">val</span><span class="p">,</span> <span class="mi">2</span><span class="p">)</span>
</pre></div>
</div>
<p>Note that when deferred fields are accessed, the loading of the deferred
field’s value happens through this method. Thus it is possible to customize
the way deferred loading happens. The example below shows how one can reload
all of the instance’s fields when a deferred field is reloaded:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">ExampleModel</span><span class="p">(</span><span class="n">models</span><span class="o">.</span><span class="n">Model</span><span class="p">):</span>
<span class="k">def</span> <span class="nf">refresh_from_db</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">using</span><span class="o">=</span><span class="kc">None</span><span class="p">,</span> <span class="n">fields</span><span class="o">=</span><span class="kc">None</span><span class="p">,</span> <span class="o">**</span><span class="n">kwargs</span><span class="p">):</span>
<span class="c1"># fields contains the name of the deferred field to be</span>
<span class="c1"># loaded.</span>
<span class="k">if</span> <span class="n">fields</span> <span class="ow">is</span> <span class="ow">not</span> <span class="kc">None</span><span class="p">:</span>
<span class="n">fields</span> <span class="o">=</span> <span class="nb">set</span><span class="p">(</span><span class="n">fields</span><span class="p">)</span>
<span class="n">deferred_fields</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">get_deferred_fields</span><span class="p">()</span>
<span class="c1"># If any deferred field is going to be loaded</span>
<span class="k">if</span> <span class="n">fields</span><span class="o">.</span><span class="n">intersection</span><span class="p">(</span><span class="n">deferred_fields</span><span class="p">):</span>
<span class="c1"># then load all of them</span>
<span class="n">fields</span> <span class="o">=</span> <span class="n">fields</span><span class="o">.</span><span class="n">union</span><span class="p">(</span><span class="n">deferred_fields</span><span class="p">)</span>
<span class="nb">super</span><span class="p">(</span><span class="n">ExampleModel</span><span class="p">,</span> <span class="bp">self</span><span class="p">)</span><span class="o">.</span><span class="n">refresh_from_db</span><span class="p">(</span><span class="n">using</span><span class="p">,</span> <span class="n">fields</span><span class="p">,</span> <span class="o">**</span><span class="n">kwargs</span><span class="p">)</span>
</pre></div>
</div>
<dl class="method">
<dt id="django.db.models.Model.get_deferred_fields">
<code class="descclassname">Model.</code><code class="descname">get_deferred_fields</code>()<a class="reference internal" href="../../_modules/django/db/models/base.html#Model.get_deferred_fields"><span class="viewcode-link">[source]</span></a><a class="headerlink" href="#django.db.models.Model.get_deferred_fields" title="Permalink to this definition">¶</a></dt>
<dd></dd></dl>
<p>A helper method that returns a set containing the attribute names of all those
fields that are currently deferred for this model.</p>
</div>
<div class="section" id="s-validating-objects">
<span id="s-id1"></span><span id="validating-objects"></span><span id="id1"></span><h2>Validating objects<a class="headerlink" href="#validating-objects" title="Permalink to this headline">¶</a></h2>
<p>There are three steps involved in validating a model:</p>
<ol class="arabic simple">
<li>Validate the model fields - <a class="reference internal" href="#django.db.models.Model.clean_fields" title="django.db.models.Model.clean_fields"><code class="xref py py-meth docutils literal notranslate"><span class="pre">Model.clean_fields()</span></code></a></li>
<li>Validate the model as a whole - <a class="reference internal" href="#django.db.models.Model.clean" title="django.db.models.Model.clean"><code class="xref py py-meth docutils literal notranslate"><span class="pre">Model.clean()</span></code></a></li>
<li>Validate the field uniqueness - <a class="reference internal" href="#django.db.models.Model.validate_unique" title="django.db.models.Model.validate_unique"><code class="xref py py-meth docutils literal notranslate"><span class="pre">Model.validate_unique()</span></code></a></li>
</ol>
<p>All three steps are performed when you call a model’s
<a class="reference internal" href="#django.db.models.Model.full_clean" title="django.db.models.Model.full_clean"><code class="xref py py-meth docutils literal notranslate"><span class="pre">full_clean()</span></code></a> method.</p>
<p>When you use a <a class="reference internal" href="../../topics/forms/modelforms.html#django.forms.ModelForm" title="django.forms.ModelForm"><code class="xref py py-class docutils literal notranslate"><span class="pre">ModelForm</span></code></a>, the call to
<a class="reference internal" href="../forms/api.html#django.forms.Form.is_valid" title="django.forms.Form.is_valid"><code class="xref py py-meth docutils literal notranslate"><span class="pre">is_valid()</span></code></a> will perform these validation steps for
all the fields that are included on the form. See the <a class="reference internal" href="../../topics/forms/modelforms.html"><span class="doc">ModelForm
documentation</span></a> for more information. You should only
need to call a model’s <a class="reference internal" href="#django.db.models.Model.full_clean" title="django.db.models.Model.full_clean"><code class="xref py py-meth docutils literal notranslate"><span class="pre">full_clean()</span></code></a> method if you plan to handle
validation errors yourself, or if you have excluded fields from the
<a class="reference internal" href="../../topics/forms/modelforms.html#django.forms.ModelForm" title="django.forms.ModelForm"><code class="xref py py-class docutils literal notranslate"><span class="pre">ModelForm</span></code></a> that require validation.</p>
<dl class="method">
<dt id="django.db.models.Model.full_clean">
<code class="descclassname">Model.</code><code class="descname">full_clean</code>(<em>exclude=None</em>, <em>validate_unique=True</em>)<a class="reference internal" href="../../_modules/django/db/models/base.html#Model.full_clean"><span class="viewcode-link">[source]</span></a><a class="headerlink" href="#django.db.models.Model.full_clean" title="Permalink to this definition">¶</a></dt>
<dd></dd></dl>
<p>This method calls <a class="reference internal" href="#django.db.models.Model.clean_fields" title="django.db.models.Model.clean_fields"><code class="xref py py-meth docutils literal notranslate"><span class="pre">Model.clean_fields()</span></code></a>, <a class="reference internal" href="#django.db.models.Model.clean" title="django.db.models.Model.clean"><code class="xref py py-meth docutils literal notranslate"><span class="pre">Model.clean()</span></code></a>, and
<a class="reference internal" href="#django.db.models.Model.validate_unique" title="django.db.models.Model.validate_unique"><code class="xref py py-meth docutils literal notranslate"><span class="pre">Model.validate_unique()</span></code></a> (if <code class="docutils literal notranslate"><span class="pre">validate_unique</span></code> is <code class="docutils literal notranslate"><span class="pre">True</span></code>), in that
order and raises a <a class="reference internal" href="../exceptions.html#django.core.exceptions.ValidationError" title="django.core.exceptions.ValidationError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">ValidationError</span></code></a> that has a
<code class="docutils literal notranslate"><span class="pre">message_dict</span></code> attribute containing errors from all three stages.</p>
<p>The optional <code class="docutils literal notranslate"><span class="pre">exclude</span></code> argument can be used to provide a list of field names
that can be excluded from validation and cleaning.
<a class="reference internal" href="../../topics/forms/modelforms.html#django.forms.ModelForm" title="django.forms.ModelForm"><code class="xref py py-class docutils literal notranslate"><span class="pre">ModelForm</span></code></a> uses this argument to exclude fields that
aren’t present on your form from being validated since any errors raised could
not be corrected by the user.</p>
<p>Note that <code class="docutils literal notranslate"><span class="pre">full_clean()</span></code> will <em>not</em> be called automatically when you call
your model’s <a class="reference internal" href="#django.db.models.Model.save" title="django.db.models.Model.save"><code class="xref py py-meth docutils literal notranslate"><span class="pre">save()</span></code></a> method. You’ll need to call it manually
when you want to run one-step model validation for your own manually created
models. For example:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">django.core.exceptions</span> <span class="k">import</span> <span class="n">ValidationError</span>
<span class="k">try</span><span class="p">:</span>
<span class="n">article</span><span class="o">.</span><span class="n">full_clean</span><span class="p">()</span>
<span class="k">except</span> <span class="n">ValidationError</span> <span class="k">as</span> <span class="n">e</span><span class="p">:</span>
<span class="c1"># Do something based on the errors contained in e.message_dict.</span>
<span class="c1"># Display them to a user, or handle them programmatically.</span>
<span class="k">pass</span>
</pre></div>
</div>
<p>The first step <code class="docutils literal notranslate"><span class="pre">full_clean()</span></code> performs is to clean each individual field.</p>
<dl class="method">
<dt id="django.db.models.Model.clean_fields">
<code class="descclassname">Model.</code><code class="descname">clean_fields</code>(<em>exclude=None</em>)<a class="reference internal" href="../../_modules/django/db/models/base.html#Model.clean_fields"><span class="viewcode-link">[source]</span></a><a class="headerlink" href="#django.db.models.Model.clean_fields" title="Permalink to this definition">¶</a></dt>
<dd></dd></dl>
<p>This method will validate all fields on your model. The optional <code class="docutils literal notranslate"><span class="pre">exclude</span></code>
argument lets you provide a list of field names to exclude from validation. It
will raise a <a class="reference internal" href="../exceptions.html#django.core.exceptions.ValidationError" title="django.core.exceptions.ValidationError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">ValidationError</span></code></a> if any fields fail
validation.</p>
<p>The second step <code class="docutils literal notranslate"><span class="pre">full_clean()</span></code> performs is to call <a class="reference internal" href="#django.db.models.Model.clean" title="django.db.models.Model.clean"><code class="xref py py-meth docutils literal notranslate"><span class="pre">Model.clean()</span></code></a>.
This method should be overridden to perform custom validation on your model.</p>
<dl class="method">
<dt id="django.db.models.Model.clean">
<code class="descclassname">Model.</code><code class="descname">clean</code>()<a class="reference internal" href="../../_modules/django/db/models/base.html#Model.clean"><span class="viewcode-link">[source]</span></a><a class="headerlink" href="#django.db.models.Model.clean" title="Permalink to this definition">¶</a></dt>
<dd></dd></dl>
<p>This method should be used to provide custom model validation, and to modify
attributes on your model if desired. For instance, you could use it to
automatically provide a value for a field, or to do validation that requires
access to more than a single field:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">datetime</span>
<span class="kn">from</span> <span class="nn">django.core.exceptions</span> <span class="k">import</span> <span class="n">ValidationError</span>
<span class="kn">from</span> <span class="nn">django.db</span> <span class="k">import</span> <span class="n">models</span>
<span class="kn">from</span> <span class="nn">django.utils.translation</span> <span class="k">import</span> <span class="n">ugettext_lazy</span> <span class="k">as</span> <span class="n">_</span>
<span class="k">class</span> <span class="nc">Article</span><span class="p">(</span><span class="n">models</span><span class="o">.</span><span class="n">Model</span><span class="p">):</span>
<span class="o">...</span>
<span class="k">def</span> <span class="nf">clean</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="c1"># Don't allow draft entries to have a pub_date.</span>
<span class="k">if</span> <span class="bp">self</span><span class="o">.</span><span class="n">status</span> <span class="o">==</span> <span class="s1">'draft'</span> <span class="ow">and</span> <span class="bp">self</span><span class="o">.</span><span class="n">pub_date</span> <span class="ow">is</span> <span class="ow">not</span> <span class="kc">None</span><span class="p">:</span>
<span class="k">raise</span> <span class="n">ValidationError</span><span class="p">(</span><span class="n">_</span><span class="p">(</span><span class="s1">'Draft entries may not have a publication date.'</span><span class="p">))</span>
<span class="c1"># Set the pub_date for published items if it hasn't been set already.</span>
<span class="k">if</span> <span class="bp">self</span><span class="o">.</span><span class="n">status</span> <span class="o">==</span> <span class="s1">'published'</span> <span class="ow">and</span> <span class="bp">self</span><span class="o">.</span><span class="n">pub_date</span> <span class="ow">is</span> <span class="kc">None</span><span class="p">:</span>
<span class="bp">self</span><span class="o">.</span><span class="n">pub_date</span> <span class="o">=</span> <span class="n">datetime</span><span class="o">.</span><span class="n">date</span><span class="o">.</span><span class="n">today</span><span class="p">()</span>
</pre></div>
</div>
<p>Note, however, that like <a class="reference internal" href="#django.db.models.Model.full_clean" title="django.db.models.Model.full_clean"><code class="xref py py-meth docutils literal notranslate"><span class="pre">Model.full_clean()</span></code></a>, a model’s <code class="docutils literal notranslate"><span class="pre">clean()</span></code>
method is not invoked when you call your model’s <a class="reference internal" href="#django.db.models.Model.save" title="django.db.models.Model.save"><code class="xref py py-meth docutils literal notranslate"><span class="pre">save()</span></code></a> method.</p>
<p>In the above example, the <a class="reference internal" href="../exceptions.html#django.core.exceptions.ValidationError" title="django.core.exceptions.ValidationError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">ValidationError</span></code></a>
exception raised by <code class="docutils literal notranslate"><span class="pre">Model.clean()</span></code> was instantiated with a string, so it
will be stored in a special error dictionary key,
<a class="reference internal" href="../exceptions.html#django.core.exceptions.NON_FIELD_ERRORS" title="django.core.exceptions.NON_FIELD_ERRORS"><code class="xref py py-data docutils literal notranslate"><span class="pre">NON_FIELD_ERRORS</span></code></a>. This key is used for errors
that are tied to the entire model instead of to a specific field:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">django.core.exceptions</span> <span class="k">import</span> <span class="n">ValidationError</span><span class="p">,</span> <span class="n">NON_FIELD_ERRORS</span>
<span class="k">try</span><span class="p">:</span>
<span class="n">article</span><span class="o">.</span><span class="n">full_clean</span><span class="p">()</span>
<span class="k">except</span> <span class="n">ValidationError</span> <span class="k">as</span> <span class="n">e</span><span class="p">:</span>
<span class="n">non_field_errors</span> <span class="o">=</span> <span class="n">e</span><span class="o">.</span><span class="n">message_dict</span><span class="p">[</span><span class="n">NON_FIELD_ERRORS</span><span class="p">]</span>
</pre></div>
</div>
<p>To assign exceptions to a specific field, instantiate the
<a class="reference internal" href="../exceptions.html#django.core.exceptions.ValidationError" title="django.core.exceptions.ValidationError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">ValidationError</span></code></a> with a dictionary, where the
keys are the field names. We could update the previous example to assign the
error to the <code class="docutils literal notranslate"><span class="pre">pub_date</span></code> field:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">Article</span><span class="p">(</span><span class="n">models</span><span class="o">.</span><span class="n">Model</span><span class="p">):</span>
<span class="o">...</span>
<span class="k">def</span> <span class="nf">clean</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="c1"># Don't allow draft entries to have a pub_date.</span>
<span class="k">if</span> <span class="bp">self</span><span class="o">.</span><span class="n">status</span> <span class="o">==</span> <span class="s1">'draft'</span> <span class="ow">and</span> <span class="bp">self</span><span class="o">.</span><span class="n">pub_date</span> <span class="ow">is</span> <span class="ow">not</span> <span class="kc">None</span><span class="p">:</span>
<span class="k">raise</span> <span class="n">ValidationError</span><span class="p">({</span><span class="s1">'pub_date'</span><span class="p">:</span> <span class="n">_</span><span class="p">(</span><span class="s1">'Draft entries may not have a publication date.'</span><span class="p">)})</span>
<span class="o">...</span>
</pre></div>
</div>
<p>If you detect errors in multiple fields during <code class="docutils literal notranslate"><span class="pre">Model.clean()</span></code>, you can also
pass a dictionary mapping field names to errors:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">raise</span> <span class="n">ValidationError</span><span class="p">({</span>
<span class="s1">'title'</span><span class="p">:</span> <span class="n">ValidationError</span><span class="p">(</span><span class="n">_</span><span class="p">(</span><span class="s1">'Missing title.'</span><span class="p">),</span> <span class="n">code</span><span class="o">=</span><span class="s1">'required'</span><span class="p">),</span>
<span class="s1">'pub_date'</span><span class="p">:</span> <span class="n">ValidationError</span><span class="p">(</span><span class="n">_</span><span class="p">(</span><span class="s1">'Invalid date.'</span><span class="p">),</span> <span class="n">code</span><span class="o">=</span><span class="s1">'invalid'</span><span class="p">),</span>
<span class="p">})</span>
</pre></div>
</div>
<p>Finally, <code class="docutils literal notranslate"><span class="pre">full_clean()</span></code> will check any unique constraints on your model.</p>
<div class="admonition-how-to-raise-field-specific-validation-errors-if-those-fields-don-t-appear-in-a-modelform admonition">
<p class="first admonition-title">How to raise field-specific validation errors if those fields don’t appear in a <code class="docutils literal notranslate"><span class="pre">ModelForm</span></code></p>
<p>You can’t raise validation errors in <code class="docutils literal notranslate"><span class="pre">Model.clean()</span></code> for fields that
don’t appear in a model form (a form may limit its fields using
<code class="docutils literal notranslate"><span class="pre">Meta.fields</span></code> or <code class="docutils literal notranslate"><span class="pre">Meta.exclude</span></code>). Doing so will raise a <code class="docutils literal notranslate"><span class="pre">ValueError</span></code>
because the validation error won’t be able to be associated with the
excluded field.</p>
<p>To work around this dilemma, instead override <a class="reference internal" href="#django.db.models.Model.clean_fields" title="django.db.models.Model.clean_fields"><code class="xref py py-meth docutils literal notranslate"><span class="pre">Model.clean_fields()</span></code></a> as it receives the list of fields
that are excluded from validation. For example:</p>
<div class="last highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">Article</span><span class="p">(</span><span class="n">models</span><span class="o">.</span><span class="n">Model</span><span class="p">):</span>
<span class="o">...</span>
<span class="k">def</span> <span class="nf">clean_fields</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">exclude</span><span class="o">=</span><span class="kc">None</span><span class="p">):</span>
<span class="nb">super</span><span class="p">(</span><span class="n">Article</span><span class="p">,</span> <span class="bp">self</span><span class="p">)</span><span class="o">.</span><span class="n">clean_fields</span><span class="p">(</span><span class="n">exclude</span><span class="o">=</span><span class="n">exclude</span><span class="p">)</span>
<span class="k">if</span> <span class="bp">self</span><span class="o">.</span><span class="n">status</span> <span class="o">==</span> <span class="s1">'draft'</span> <span class="ow">and</span> <span class="bp">self</span><span class="o">.</span><span class="n">pub_date</span> <span class="ow">is</span> <span class="ow">not</span> <span class="kc">None</span><span class="p">:</span>
<span class="k">if</span> <span class="n">exclude</span> <span class="ow">and</span> <span class="s1">'status'</span> <span class="ow">in</span> <span class="n">exclude</span><span class="p">:</span>
<span class="k">raise</span> <span class="n">ValidationError</span><span class="p">(</span>
<span class="n">_</span><span class="p">(</span><span class="s1">'Draft entries may not have a publication date.'</span><span class="p">)</span>
<span class="p">)</span>
<span class="k">else</span><span class="p">:</span>
<span class="k">raise</span> <span class="n">ValidationError</span><span class="p">({</span>
<span class="s1">'status'</span><span class="p">:</span> <span class="n">_</span><span class="p">(</span>
<span class="s1">'Set status to draft if there is not a '</span>
<span class="s1">'publication date.'</span>
<span class="p">),</span>
<span class="p">})</span>
</pre></div>
</div>
</div>
<dl class="method">
<dt id="django.db.models.Model.validate_unique">
<code class="descclassname">Model.</code><code class="descname">validate_unique</code>(<em>exclude=None</em>)<a class="reference internal" href="../../_modules/django/db/models/base.html#Model.validate_unique"><span class="viewcode-link">[source]</span></a><a class="headerlink" href="#django.db.models.Model.validate_unique" title="Permalink to this definition">¶</a></dt>
<dd></dd></dl>
<p>This method is similar to <a class="reference internal" href="#django.db.models.Model.clean_fields" title="django.db.models.Model.clean_fields"><code class="xref py py-meth docutils literal notranslate"><span class="pre">clean_fields()</span></code></a>, but validates all
uniqueness constraints on your model instead of individual field values. The
optional <code class="docutils literal notranslate"><span class="pre">exclude</span></code> argument allows you to provide a list of field names to
exclude from validation. It will raise a
<a class="reference internal" href="../exceptions.html#django.core.exceptions.ValidationError" title="django.core.exceptions.ValidationError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">ValidationError</span></code></a> if any fields fail validation.</p>
<p>Note that if you provide an <code class="docutils literal notranslate"><span class="pre">exclude</span></code> argument to <code class="docutils literal notranslate"><span class="pre">validate_unique()</span></code>, any
<a class="reference internal" href="options.html#django.db.models.Options.unique_together" title="django.db.models.Options.unique_together"><code class="xref py py-attr docutils literal notranslate"><span class="pre">unique_together</span></code></a> constraint involving one of
the fields you provided will not be checked.</p>
</div>
<div class="section" id="s-saving-objects">
<span id="saving-objects"></span><h2>Saving objects<a class="headerlink" href="#saving-objects" title="Permalink to this headline">¶</a></h2>
<p>To save an object back to the database, call <code class="docutils literal notranslate"><span class="pre">save()</span></code>:</p>
<dl class="method">
<dt id="django.db.models.Model.save">
<code class="descclassname">Model.</code><code class="descname">save</code>(<em>force_insert=False</em>, <em>force_update=False</em>, <em>using=DEFAULT_DB_ALIAS</em>, <em>update_fields=None</em>)<a class="reference internal" href="../../_modules/django/db/models/base.html#Model.save"><span class="viewcode-link">[source]</span></a><a class="headerlink" href="#django.db.models.Model.save" title="Permalink to this definition">¶</a></dt>
<dd></dd></dl>
<p>If you want customized saving behavior, you can override this <code class="docutils literal notranslate"><span class="pre">save()</span></code>
method. See <a class="reference internal" href="../../topics/db/models.html#overriding-model-methods"><span class="std std-ref">Overriding predefined model methods</span></a> for more details.</p>
<p>The model save process also has some subtleties; see the sections below.</p>
<div class="section" id="s-auto-incrementing-primary-keys">
<span id="auto-incrementing-primary-keys"></span><h3>Auto-incrementing primary keys<a class="headerlink" href="#auto-incrementing-primary-keys" title="Permalink to this headline">¶</a></h3>
<p>If a model has an <a class="reference internal" href="fields.html#django.db.models.AutoField" title="django.db.models.AutoField"><code class="xref py py-class docutils literal notranslate"><span class="pre">AutoField</span></code></a> — an auto-incrementing
primary key — then that auto-incremented value will be calculated and saved as
an attribute on your object the first time you call <code class="docutils literal notranslate"><span class="pre">save()</span></code>:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">b2</span> <span class="o">=</span> <span class="n">Blog</span><span class="p">(</span><span class="n">name</span><span class="o">=</span><span class="s1">'Cheddar Talk'</span><span class="p">,</span> <span class="n">tagline</span><span class="o">=</span><span class="s1">'Thoughts on cheese.'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">b2</span><span class="o">.</span><span class="n">id</span> <span class="c1"># Returns None, because b doesn't have an ID yet.</span>
<span class="gp">>>> </span><span class="n">b2</span><span class="o">.</span><span class="n">save</span><span class="p">()</span>
<span class="gp">>>> </span><span class="n">b2</span><span class="o">.</span><span class="n">id</span> <span class="c1"># Returns the ID of your new object.</span>
</pre></div>
</div>
<p>There’s no way to tell what the value of an ID will be before you call
<code class="docutils literal notranslate"><span class="pre">save()</span></code>, because that value is calculated by your database, not by Django.</p>
<p>For convenience, each model has an <a class="reference internal" href="fields.html#django.db.models.AutoField" title="django.db.models.AutoField"><code class="xref py py-class docutils literal notranslate"><span class="pre">AutoField</span></code></a> named
<code class="docutils literal notranslate"><span class="pre">id</span></code> by default unless you explicitly specify <code class="docutils literal notranslate"><span class="pre">primary_key=True</span></code> on a field
in your model. See the documentation for <a class="reference internal" href="fields.html#django.db.models.AutoField" title="django.db.models.AutoField"><code class="xref py py-class docutils literal notranslate"><span class="pre">AutoField</span></code></a>
for more details.</p>
<div class="section" id="s-the-pk-property">
<span id="the-pk-property"></span><h4>The <code class="docutils literal notranslate"><span class="pre">pk</span></code> property<a class="headerlink" href="#the-pk-property" title="Permalink to this headline">¶</a></h4>
<dl class="attribute">
<dt id="django.db.models.Model.pk">
<code class="descclassname">Model.</code><code class="descname">pk</code><a class="headerlink" href="#django.db.models.Model.pk" title="Permalink to this definition">¶</a></dt>
<dd></dd></dl>
<p>Regardless of whether you define a primary key field yourself, or let Django
supply one for you, each model will have a property called <code class="docutils literal notranslate"><span class="pre">pk</span></code>. It behaves
like a normal attribute on the model, but is actually an alias for whichever
attribute is the primary key field for the model. You can read and set this
value, just as you would for any other attribute, and it will update the
correct field in the model.</p>
</div>
<div class="section" id="s-explicitly-specifying-auto-primary-key-values">
<span id="explicitly-specifying-auto-primary-key-values"></span><h4>Explicitly specifying auto-primary-key values<a class="headerlink" href="#explicitly-specifying-auto-primary-key-values" title="Permalink to this headline">¶</a></h4>
<p>If a model has an <a class="reference internal" href="fields.html#django.db.models.AutoField" title="django.db.models.AutoField"><code class="xref py py-class docutils literal notranslate"><span class="pre">AutoField</span></code></a> but you want to define a
new object’s ID explicitly when saving, just define it explicitly before
saving, rather than relying on the auto-assignment of the ID:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">b3</span> <span class="o">=</span> <span class="n">Blog</span><span class="p">(</span><span class="nb">id</span><span class="o">=</span><span class="mi">3</span><span class="p">,</span> <span class="n">name</span><span class="o">=</span><span class="s1">'Cheddar Talk'</span><span class="p">,</span> <span class="n">tagline</span><span class="o">=</span><span class="s1">'Thoughts on cheese.'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">b3</span><span class="o">.</span><span class="n">id</span> <span class="c1"># Returns 3.</span>
<span class="gp">>>> </span><span class="n">b3</span><span class="o">.</span><span class="n">save</span><span class="p">()</span>
<span class="gp">>>> </span><span class="n">b3</span><span class="o">.</span><span class="n">id</span> <span class="c1"># Returns 3.</span>
</pre></div>
</div>
<p>If you assign auto-primary-key values manually, make sure not to use an
already-existing primary-key value! If you create a new object with an explicit
primary-key value that already exists in the database, Django will assume you’re
changing the existing record rather than creating a new one.</p>
<p>Given the above <code class="docutils literal notranslate"><span class="pre">'Cheddar</span> <span class="pre">Talk'</span></code> blog example, this example would override the
previous record in the database:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">b4</span> <span class="o">=</span> <span class="n">Blog</span><span class="p">(</span><span class="nb">id</span><span class="o">=</span><span class="mi">3</span><span class="p">,</span> <span class="n">name</span><span class="o">=</span><span class="s1">'Not Cheddar'</span><span class="p">,</span> <span class="n">tagline</span><span class="o">=</span><span class="s1">'Anything but cheese.'</span><span class="p">)</span>
<span class="n">b4</span><span class="o">.</span><span class="n">save</span><span class="p">()</span> <span class="c1"># Overrides the previous blog with ID=3!</span>
</pre></div>
</div>
<p>See <a class="reference internal" href="#how-django-knows-to-update-vs-insert">How Django knows to UPDATE vs. INSERT</a>, below, for the reason this
happens.</p>
<p>Explicitly specifying auto-primary-key values is mostly useful for bulk-saving
objects, when you’re confident you won’t have primary-key collision.</p>
<p>If you’re using PostgreSQL, the sequence associated with the primary key might
need to be updated; see <a class="reference internal" href="../databases.html#manually-specified-autoincrement-pk"><span class="std std-ref">Manually-specifying values of auto-incrementing primary keys</span></a>.</p>
</div>
</div>
<div class="section" id="s-what-happens-when-you-save">
<span id="what-happens-when-you-save"></span><h3>What happens when you save?<a class="headerlink" href="#what-happens-when-you-save" title="Permalink to this headline">¶</a></h3>
<p>When you save an object, Django performs the following steps:</p>
<ol class="arabic">
<li><p class="first"><strong>Emit a pre-save signal.</strong> The <a class="reference internal" href="../signals.html#django.db.models.signals.pre_save" title="django.db.models.signals.pre_save"><code class="xref py py-data docutils literal notranslate"><span class="pre">pre_save</span></code></a>
signal is sent, allowing any functions listening for that signal to do
something.</p>
</li>
<li><p class="first"><strong>Preprocess the data.</strong> Each field’s
<a class="reference internal" href="fields.html#django.db.models.Field.pre_save" title="django.db.models.Field.pre_save"><code class="xref py py-meth docutils literal notranslate"><span class="pre">pre_save()</span></code></a> method is called to perform any
automated data modification that’s needed. For example, the date/time fields
override <code class="docutils literal notranslate"><span class="pre">pre_save()</span></code> to implement
<a class="reference internal" href="fields.html#django.db.models.DateField.auto_now_add" title="django.db.models.DateField.auto_now_add"><code class="xref py py-attr docutils literal notranslate"><span class="pre">auto_now_add</span></code></a> and
<a class="reference internal" href="fields.html#django.db.models.DateField.auto_now" title="django.db.models.DateField.auto_now"><code class="xref py py-attr docutils literal notranslate"><span class="pre">auto_now</span></code></a>.</p>
</li>
<li><p class="first"><strong>Prepare the data for the database.</strong> Each field’s
<a class="reference internal" href="fields.html#django.db.models.Field.get_db_prep_save" title="django.db.models.Field.get_db_prep_save"><code class="xref py py-meth docutils literal notranslate"><span class="pre">get_db_prep_save()</span></code></a> method is asked to provide
its current value in a data type that can be written to the database.</p>
<p>Most fields don’t require data preparation. Simple data types, such as
integers and strings, are ‘ready to write’ as a Python object. However, more
complex data types often require some modification.</p>
<p>For example, <a class="reference internal" href="fields.html#django.db.models.DateField" title="django.db.models.DateField"><code class="xref py py-class docutils literal notranslate"><span class="pre">DateField</span></code></a> fields use a Python
<code class="docutils literal notranslate"><span class="pre">datetime</span></code> object to store data. Databases don’t store <code class="docutils literal notranslate"><span class="pre">datetime</span></code>
objects, so the field value must be converted into an ISO-compliant date
string for insertion into the database.</p>
</li>
<li><p class="first"><strong>Insert the data into the database.</strong> The preprocessed, prepared data is
composed into an SQL statement for insertion into the database.</p>
</li>
<li><p class="first"><strong>Emit a post-save signal.</strong> The <a class="reference internal" href="../signals.html#django.db.models.signals.post_save" title="django.db.models.signals.post_save"><code class="xref py py-data docutils literal notranslate"><span class="pre">post_save</span></code></a>
signal is sent, allowing any functions listening for that signal to do
something.</p>
</li>
</ol>
</div>
<div class="section" id="s-how-django-knows-to-update-vs-insert">
<span id="how-django-knows-to-update-vs-insert"></span><h3>How Django knows to UPDATE vs. INSERT<a class="headerlink" href="#how-django-knows-to-update-vs-insert" title="Permalink to this headline">¶</a></h3>
<p>You may have noticed Django database objects use the same <code class="docutils literal notranslate"><span class="pre">save()</span></code> method
for creating and changing objects. Django abstracts the need to use <code class="docutils literal notranslate"><span class="pre">INSERT</span></code>
or <code class="docutils literal notranslate"><span class="pre">UPDATE</span></code> SQL statements. Specifically, when you call <code class="docutils literal notranslate"><span class="pre">save()</span></code>, Django
follows this algorithm:</p>
<ul class="simple">
<li>If the object’s primary key attribute is set to a value that evaluates to
<code class="docutils literal notranslate"><span class="pre">True</span></code> (i.e., a value other than <code class="docutils literal notranslate"><span class="pre">None</span></code> or the empty string), Django
executes an <code class="docutils literal notranslate"><span class="pre">UPDATE</span></code>.</li>
<li>If the object’s primary key attribute is <em>not</em> set or if the <code class="docutils literal notranslate"><span class="pre">UPDATE</span></code>
didn’t update anything, Django executes an <code class="docutils literal notranslate"><span class="pre">INSERT</span></code>.</li>
</ul>
<p>The one gotcha here is that you should be careful not to specify a primary-key
value explicitly when saving new objects, if you cannot guarantee the
primary-key value is unused. For more on this nuance, see <a class="reference internal" href="#explicitly-specifying-auto-primary-key-values">Explicitly specifying
auto-primary-key values</a> above and <a class="reference internal" href="#forcing-an-insert-or-update">Forcing an INSERT or UPDATE</a> below.</p>
<p>In Django 1.5 and earlier, Django did a <code class="docutils literal notranslate"><span class="pre">SELECT</span></code> when the primary key
attribute was set. If the <code class="docutils literal notranslate"><span class="pre">SELECT</span></code> found a row, then Django did an <code class="docutils literal notranslate"><span class="pre">UPDATE</span></code>,
otherwise it did an <code class="docutils literal notranslate"><span class="pre">INSERT</span></code>. The old algorithm results in one more query in
the <code class="docutils literal notranslate"><span class="pre">UPDATE</span></code> case. There are some rare cases where the database doesn’t
report that a row was updated even if the database contains a row for the
object’s primary key value. An example is the PostgreSQL <code class="docutils literal notranslate"><span class="pre">ON</span> <span class="pre">UPDATE</span></code> trigger
which returns <code class="docutils literal notranslate"><span class="pre">NULL</span></code>. In such cases it is possible to revert to the old
algorithm by setting the <a class="reference internal" href="options.html#django.db.models.Options.select_on_save" title="django.db.models.Options.select_on_save"><code class="xref py py-attr docutils literal notranslate"><span class="pre">select_on_save</span></code></a>
option to <code class="docutils literal notranslate"><span class="pre">True</span></code>.</p>
<div class="section" id="s-forcing-an-insert-or-update">
<span id="s-ref-models-force-insert"></span><span id="forcing-an-insert-or-update"></span><span id="ref-models-force-insert"></span><h4>Forcing an INSERT or UPDATE<a class="headerlink" href="#forcing-an-insert-or-update" title="Permalink to this headline">¶</a></h4>
<p>In some rare circumstances, it’s necessary to be able to force the
<a class="reference internal" href="#django.db.models.Model.save" title="django.db.models.Model.save"><code class="xref py py-meth docutils literal notranslate"><span class="pre">save()</span></code></a> method to perform an SQL <code class="docutils literal notranslate"><span class="pre">INSERT</span></code> and not fall back to
doing an <code class="docutils literal notranslate"><span class="pre">UPDATE</span></code>. Or vice-versa: update, if possible, but not insert a new
row. In these cases you can pass the <code class="docutils literal notranslate"><span class="pre">force_insert=True</span></code> or
<code class="docutils literal notranslate"><span class="pre">force_update=True</span></code> parameters to the <a class="reference internal" href="#django.db.models.Model.save" title="django.db.models.Model.save"><code class="xref py py-meth docutils literal notranslate"><span class="pre">save()</span></code></a> method.
Obviously, passing both parameters is an error: you cannot both insert <em>and</em>
update at the same time!</p>
<p>It should be very rare that you’ll need to use these parameters. Django will
almost always do the right thing and trying to override that will lead to
errors that are difficult to track down. This feature is for advanced use
only.</p>
<p>Using <code class="docutils literal notranslate"><span class="pre">update_fields</span></code> will force an update similarly to <code class="docutils literal notranslate"><span class="pre">force_update</span></code>.</p>
</div>
</div>
<div class="section" id="s-updating-attributes-based-on-existing-fields">
<span id="s-ref-models-field-updates-using-f-expressions"></span><span id="updating-attributes-based-on-existing-fields"></span><span id="ref-models-field-updates-using-f-expressions"></span><h3>Updating attributes based on existing fields<a class="headerlink" href="#updating-attributes-based-on-existing-fields" title="Permalink to this headline">¶</a></h3>
<p>Sometimes you’ll need to perform a simple arithmetic task on a field, such
as incrementing or decrementing the current value. The obvious way to
achieve this is to do something like:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">product</span> <span class="o">=</span> <span class="n">Product</span><span class="o">.</span><span class="n">objects</span><span class="o">.</span><span class="n">get</span><span class="p">(</span><span class="n">name</span><span class="o">=</span><span class="s1">'Venezuelan Beaver Cheese'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">product</span><span class="o">.</span><span class="n">number_sold</span> <span class="o">+=</span> <span class="mi">1</span>
<span class="gp">>>> </span><span class="n">product</span><span class="o">.</span><span class="n">save</span><span class="p">()</span>
</pre></div>
</div>
<p>If the old <code class="docutils literal notranslate"><span class="pre">number_sold</span></code> value retrieved from the database was 10, then
the value of 11 will be written back to the database.</p>
<p>The process can be made robust, <a class="reference internal" href="expressions.html#avoiding-race-conditions-using-f"><span class="std std-ref">avoiding a race condition</span></a>, as well as slightly faster by expressing
the update relative to the original field value, rather than as an explicit
assignment of a new value. Django provides <a class="reference internal" href="expressions.html#django.db.models.F" title="django.db.models.F"><code class="xref py py-class docutils literal notranslate"><span class="pre">F</span> <span class="pre">expressions</span></code></a> for performing this kind of relative update. Using
<a class="reference internal" href="expressions.html#django.db.models.F" title="django.db.models.F"><code class="xref py py-class docutils literal notranslate"><span class="pre">F</span> <span class="pre">expressions</span></code></a>, the previous example is expressed
as:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="kn">from</span> <span class="nn">django.db.models</span> <span class="k">import</span> <span class="n">F</span>
<span class="gp">>>> </span><span class="n">product</span> <span class="o">=</span> <span class="n">Product</span><span class="o">.</span><span class="n">objects</span><span class="o">.</span><span class="n">get</span><span class="p">(</span><span class="n">name</span><span class="o">=</span><span class="s1">'Venezuelan Beaver Cheese'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">product</span><span class="o">.</span><span class="n">number_sold</span> <span class="o">=</span> <span class="n">F</span><span class="p">(</span><span class="s1">'number_sold'</span><span class="p">)</span> <span class="o">+</span> <span class="mi">1</span>
<span class="gp">>>> </span><span class="n">product</span><span class="o">.</span><span class="n">save</span><span class="p">()</span>
</pre></div>
</div>
<p>For more details, see the documentation on <a class="reference internal" href="expressions.html#django.db.models.F" title="django.db.models.F"><code class="xref py py-class docutils literal notranslate"><span class="pre">F</span> <span class="pre">expressions</span></code></a> and their <a class="reference internal" href="../../topics/db/queries.html#topics-db-queries-update"><span class="std std-ref">use in update queries</span></a>.</p>
</div>
<div class="section" id="s-specifying-which-fields-to-save">
<span id="specifying-which-fields-to-save"></span><h3>Specifying which fields to save<a class="headerlink" href="#specifying-which-fields-to-save" title="Permalink to this headline">¶</a></h3>
<p>If <code class="docutils literal notranslate"><span class="pre">save()</span></code> is passed a list of field names in keyword argument
<code class="docutils literal notranslate"><span class="pre">update_fields</span></code>, only the fields named in that list will be updated.
This may be desirable if you want to update just one or a few fields on
an object. There will be a slight performance benefit from preventing
all of the model fields from being updated in the database. For example:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">product</span><span class="o">.</span><span class="n">name</span> <span class="o">=</span> <span class="s1">'Name changed again'</span>
<span class="n">product</span><span class="o">.</span><span class="n">save</span><span class="p">(</span><span class="n">update_fields</span><span class="o">=</span><span class="p">[</span><span class="s1">'name'</span><span class="p">])</span>
</pre></div>
</div>
<p>The <code class="docutils literal notranslate"><span class="pre">update_fields</span></code> argument can be any iterable containing strings. An
empty <code class="docutils literal notranslate"><span class="pre">update_fields</span></code> iterable will skip the save. A value of None will
perform an update on all fields.</p>
<p>Specifying <code class="docutils literal notranslate"><span class="pre">update_fields</span></code> will force an update.</p>
<p>When saving a model fetched through deferred model loading
(<a class="reference internal" href="querysets.html#django.db.models.query.QuerySet.only" title="django.db.models.query.QuerySet.only"><code class="xref py py-meth docutils literal notranslate"><span class="pre">only()</span></code></a> or
<a class="reference internal" href="querysets.html#django.db.models.query.QuerySet.defer" title="django.db.models.query.QuerySet.defer"><code class="xref py py-meth docutils literal notranslate"><span class="pre">defer()</span></code></a>) only the fields loaded
from the DB will get updated. In effect there is an automatic
<code class="docutils literal notranslate"><span class="pre">update_fields</span></code> in this case. If you assign or change any deferred field
value, the field will be added to the updated fields.</p>
</div>
</div>
<div class="section" id="s-deleting-objects">
<span id="deleting-objects"></span><h2>Deleting objects<a class="headerlink" href="#deleting-objects" title="Permalink to this headline">¶</a></h2>
<dl class="method">
<dt id="django.db.models.Model.delete">
<code class="descclassname">Model.</code><code class="descname">delete</code>(<em>using=DEFAULT_DB_ALIAS</em>, <em>keep_parents=False</em>)<a class="reference internal" href="../../_modules/django/db/models/base.html#Model.delete"><span class="viewcode-link">[source]</span></a><a class="headerlink" href="#django.db.models.Model.delete" title="Permalink to this definition">¶</a></dt>
<dd></dd></dl>
<p>Issues an SQL <code class="docutils literal notranslate"><span class="pre">DELETE</span></code> for the object. This only deletes the object in the
database; the Python instance will still exist and will still have data in
its fields. This method returns the number of objects deleted and a dictionary
with the number of deletions per object type.</p>
<p>For more details, including how to delete objects in bulk, see
<a class="reference internal" href="../../topics/db/queries.html#topics-db-queries-delete"><span class="std std-ref">Deleting objects</span></a>.</p>
<p>If you want customized deletion behavior, you can override the <code class="docutils literal notranslate"><span class="pre">delete()</span></code>
method. See <a class="reference internal" href="../../topics/db/models.html#overriding-model-methods"><span class="std std-ref">Overriding predefined model methods</span></a> for more details.</p>
<p>Sometimes with <a class="reference internal" href="../../topics/db/models.html#multi-table-inheritance"><span class="std std-ref">multi-table inheritance</span></a> you may
want to delete only a child model’s data. Specifying <code class="docutils literal notranslate"><span class="pre">keep_parents=True</span></code> will
keep the parent model’s data.</p>
</div>
<div class="section" id="s-pickling-objects">
<span id="pickling-objects"></span><h2>Pickling objects<a class="headerlink" href="#pickling-objects" title="Permalink to this headline">¶</a></h2>
<p>When you <code class="xref py py-mod docutils literal notranslate"><span class="pre">pickle</span></code> a model, its current state is pickled. When you unpickle
it, it’ll contain the model instance at the moment it was pickled, rather than
the data that’s currently in the database.</p>
<div class="admonition-you-can-t-share-pickles-between-versions admonition">
<p class="first admonition-title">You can’t share pickles between versions</p>
<p>Pickles of models are only valid for the version of Django that
was used to generate them. If you generate a pickle using Django
version N, there is no guarantee that pickle will be readable with
Django version N+1. Pickles should not be used as part of a long-term
archival strategy.</p>
<p class="last">Since pickle compatibility errors can be difficult to diagnose, such as
silently corrupted objects, a <code class="docutils literal notranslate"><span class="pre">RuntimeWarning</span></code> is raised when you try to
unpickle a model in a Django version that is different than the one in
which it was pickled.</p>
</div>
</div>
<div class="section" id="s-other-model-instance-methods">
<span id="s-model-instance-methods"></span><span id="other-model-instance-methods"></span><span id="model-instance-methods"></span><h2>Other model instance methods<a class="headerlink" href="#other-model-instance-methods" title="Permalink to this headline">¶</a></h2>
<p>A few object methods have special purposes.</p>
<div class="section" id="s-str">
<span id="str"></span><h3><code class="docutils literal notranslate"><span class="pre">__str__()</span></code><a class="headerlink" href="#str" title="Permalink to this headline">¶</a></h3>
<dl class="method">
<dt id="django.db.models.Model.__str__">
<code class="descclassname">Model.</code><code class="descname">__str__</code>()<a class="reference internal" href="../../_modules/django/db/models/base.html#Model.__str__"><span class="viewcode-link">[source]</span></a><a class="headerlink" href="#django.db.models.Model.__str__" title="Permalink to this definition">¶</a></dt>
<dd></dd></dl>
<p>The <code class="docutils literal notranslate"><span class="pre">__str__()</span></code> method is called whenever you call <code class="docutils literal notranslate"><span class="pre">str()</span></code> on an object.
Django uses <code class="docutils literal notranslate"><span class="pre">str(obj)</span></code> in a number of places. Most notably, to display an
object in the Django admin site and as the value inserted into a template when
it displays an object. Thus, you should always return a nice, human-readable
representation of the model from the <code class="docutils literal notranslate"><span class="pre">__str__()</span></code> method.</p>
<p>For example:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">django.db</span> <span class="k">import</span> <span class="n">models</span>
<span class="kn">from</span> <span class="nn">django.utils.encoding</span> <span class="k">import</span> <span class="n">python_2_unicode_compatible</span>
<span class="nd">@python_2_unicode_compatible</span> <span class="c1"># only if you need to support Python 2</span>
<span class="k">class</span> <span class="nc">Person</span><span class="p">(</span><span class="n">models</span><span class="o">.</span><span class="n">Model</span><span class="p">):</span>
<span class="n">first_name</span> <span class="o">=</span> <span class="n">models</span><span class="o">.</span><span class="n">CharField</span><span class="p">(</span><span class="n">max_length</span><span class="o">=</span><span class="mi">50</span><span class="p">)</span>
<span class="n">last_name</span> <span class="o">=</span> <span class="n">models</span><span class="o">.</span><span class="n">CharField</span><span class="p">(</span><span class="n">max_length</span><span class="o">=</span><span class="mi">50</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">__str__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="k">return</span> <span class="s1">'</span><span class="si">%s</span><span class="s1"> </span><span class="si">%s</span><span class="s1">'</span> <span class="o">%</span> <span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">first_name</span><span class="p">,</span> <span class="bp">self</span><span class="o">.</span><span class="n">last_name</span><span class="p">)</span>
</pre></div>
</div>
<p>If you’d like compatibility with Python 2, you can decorate your model class
with <a class="reference internal" href="../utils.html#django.utils.encoding.python_2_unicode_compatible" title="django.utils.encoding.python_2_unicode_compatible"><code class="xref py py-func docutils literal notranslate"><span class="pre">python_2_unicode_compatible()</span></code></a> as shown above.</p>
</div>
<div class="section" id="s-eq">
<span id="eq"></span><h3><code class="docutils literal notranslate"><span class="pre">__eq__()</span></code><a class="headerlink" href="#eq" title="Permalink to this headline">¶</a></h3>
<dl class="method">
<dt id="django.db.models.Model.__eq__">
<code class="descclassname">Model.</code><code class="descname">__eq__</code>()<a class="reference internal" href="../../_modules/django/db/models/base.html#Model.__eq__"><span class="viewcode-link">[source]</span></a><a class="headerlink" href="#django.db.models.Model.__eq__" title="Permalink to this definition">¶</a></dt>
<dd></dd></dl>
<p>The equality method is defined such that instances with the same primary
key value and the same concrete class are considered equal, except that
instances with a primary key value of <code class="docutils literal notranslate"><span class="pre">None</span></code> aren’t equal to anything except
themselves. For proxy models, concrete class is defined as the model’s first
non-proxy parent; for all other models it’s simply the model’s class.</p>
<p>For example:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">django.db</span> <span class="k">import</span> <span class="n">models</span>
<span class="k">class</span> <span class="nc">MyModel</span><span class="p">(</span><span class="n">models</span><span class="o">.</span><span class="n">Model</span><span class="p">):</span>
<span class="nb">id</span> <span class="o">=</span> <span class="n">models</span><span class="o">.</span><span class="n">AutoField</span><span class="p">(</span><span class="n">primary_key</span><span class="o">=</span><span class="kc">True</span><span class="p">)</span>
<span class="k">class</span> <span class="nc">MyProxyModel</span><span class="p">(</span><span class="n">MyModel</span><span class="p">):</span>
<span class="k">class</span> <span class="nc">Meta</span><span class="p">:</span>
<span class="n">proxy</span> <span class="o">=</span> <span class="kc">True</span>
<span class="k">class</span> <span class="nc">MultitableInherited</span><span class="p">(</span><span class="n">MyModel</span><span class="p">):</span>
<span class="k">pass</span>
<span class="c1"># Primary keys compared</span>
<span class="n">MyModel</span><span class="p">(</span><span class="nb">id</span><span class="o">=</span><span class="mi">1</span><span class="p">)</span> <span class="o">==</span> <span class="n">MyModel</span><span class="p">(</span><span class="nb">id</span><span class="o">=</span><span class="mi">1</span><span class="p">)</span>
<span class="n">MyModel</span><span class="p">(</span><span class="nb">id</span><span class="o">=</span><span class="mi">1</span><span class="p">)</span> <span class="o">!=</span> <span class="n">MyModel</span><span class="p">(</span><span class="nb">id</span><span class="o">=</span><span class="mi">2</span><span class="p">)</span>
<span class="c1"># Primay keys are None</span>
<span class="n">MyModel</span><span class="p">(</span><span class="nb">id</span><span class="o">=</span><span class="kc">None</span><span class="p">)</span> <span class="o">!=</span> <span class="n">MyModel</span><span class="p">(</span><span class="nb">id</span><span class="o">=</span><span class="kc">None</span><span class="p">)</span>
<span class="c1"># Same instance</span>
<span class="n">instance</span> <span class="o">=</span> <span class="n">MyModel</span><span class="p">(</span><span class="nb">id</span><span class="o">=</span><span class="kc">None</span><span class="p">)</span>
<span class="n">instance</span> <span class="o">==</span> <span class="n">instance</span>
<span class="c1"># Proxy model</span>
<span class="n">MyModel</span><span class="p">(</span><span class="nb">id</span><span class="o">=</span><span class="mi">1</span><span class="p">)</span> <span class="o">==</span> <span class="n">MyProxyModel</span><span class="p">(</span><span class="nb">id</span><span class="o">=</span><span class="mi">1</span><span class="p">)</span>
<span class="c1"># Multi-table inheritance</span>
<span class="n">MyModel</span><span class="p">(</span><span class="nb">id</span><span class="o">=</span><span class="mi">1</span><span class="p">)</span> <span class="o">!=</span> <span class="n">MultitableInherited</span><span class="p">(</span><span class="nb">id</span><span class="o">=</span><span class="mi">1</span><span class="p">)</span>
</pre></div>
</div>
</div>
<div class="section" id="s-hash">
<span id="hash"></span><h3><code class="docutils literal notranslate"><span class="pre">__hash__()</span></code><a class="headerlink" href="#hash" title="Permalink to this headline">¶</a></h3>
<dl class="method">
<dt id="django.db.models.Model.__hash__">
<code class="descclassname">Model.</code><code class="descname">__hash__</code>()<a class="reference internal" href="../../_modules/django/db/models/base.html#Model.__hash__"><span class="viewcode-link">[source]</span></a><a class="headerlink" href="#django.db.models.Model.__hash__" title="Permalink to this definition">¶</a></dt>
<dd></dd></dl>
<p>The <code class="docutils literal notranslate"><span class="pre">__hash__()</span></code> method is based on the instance’s primary key value. It
is effectively <code class="docutils literal notranslate"><span class="pre">hash(obj.pk)</span></code>. If the instance doesn’t have a primary key
value then a <code class="docutils literal notranslate"><span class="pre">TypeError</span></code> will be raised (otherwise the <code class="docutils literal notranslate"><span class="pre">__hash__()</span></code>
method would return different values before and after the instance is
saved, but changing the <code class="xref py py-meth docutils literal notranslate"><span class="pre">__hash__()</span></code> value of an instance is
forbidden in Python.</p>
</div>
<div class="section" id="s-get-absolute-url">
<span id="get-absolute-url"></span><h3><code class="docutils literal notranslate"><span class="pre">get_absolute_url()</span></code><a class="headerlink" href="#get-absolute-url" title="Permalink to this headline">¶</a></h3>
<dl class="method">
<dt id="django.db.models.Model.get_absolute_url">
<code class="descclassname">Model.</code><code class="descname">get_absolute_url</code>()<a class="headerlink" href="#django.db.models.Model.get_absolute_url" title="Permalink to this definition">¶</a></dt>
<dd></dd></dl>
<p>Define a <code class="docutils literal notranslate"><span class="pre">get_absolute_url()</span></code> method to tell Django how to calculate the
canonical URL for an object. To callers, this method should appear to return a
string that can be used to refer to the object over HTTP.</p>
<p>For example:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">get_absolute_url</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="k">return</span> <span class="s2">"/people/</span><span class="si">%i</span><span class="s2">/"</span> <span class="o">%</span> <span class="bp">self</span><span class="o">.</span><span class="n">id</span>
</pre></div>
</div>
<p>While this code is correct and simple, it may not be the most portable way to
to write this kind of method. The <a class="reference internal" href="../urlresolvers.html#django.urls.reverse" title="django.urls.reverse"><code class="xref py py-func docutils literal notranslate"><span class="pre">reverse()</span></code></a> function is
usually the best approach.</p>
<p>For example:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">get_absolute_url</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="kn">from</span> <span class="nn">django.urls</span> <span class="k">import</span> <span class="n">reverse</span>
<span class="k">return</span> <span class="n">reverse</span><span class="p">(</span><span class="s1">'people.views.details'</span><span class="p">,</span> <span class="n">args</span><span class="o">=</span><span class="p">[</span><span class="nb">str</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">id</span><span class="p">)])</span>
</pre></div>
</div>
<p>One place Django uses <code class="docutils literal notranslate"><span class="pre">get_absolute_url()</span></code> is in the admin app. If an object
defines this method, the object-editing page will have a “View on site” link
that will jump you directly to the object’s public view, as given by
<code class="docutils literal notranslate"><span class="pre">get_absolute_url()</span></code>.</p>
<p>Similarly, a couple of other bits of Django, such as the <a class="reference internal" href="../contrib/syndication.html"><span class="doc">syndication feed
framework</span></a>, use <code class="docutils literal notranslate"><span class="pre">get_absolute_url()</span></code> when it is
defined. If it makes sense for your model’s instances to each have a unique
URL, you should define <code class="docutils literal notranslate"><span class="pre">get_absolute_url()</span></code>.</p>
<div class="admonition warning">
<p class="first admonition-title">Warning</p>
<p>You should avoid building the URL from unvalidated user input, in order to
reduce possibilities of link or redirect poisoning:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">get_absolute_url</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="k">return</span> <span class="s1">'/</span><span class="si">%s</span><span class="s1">/'</span> <span class="o">%</span> <span class="bp">self</span><span class="o">.</span><span class="n">name</span>
</pre></div>
</div>
<p class="last">If <code class="docutils literal notranslate"><span class="pre">self.name</span></code> is <code class="docutils literal notranslate"><span class="pre">'/example.com'</span></code> this returns <code class="docutils literal notranslate"><span class="pre">'//example.com/'</span></code>
which, in turn, is a valid schema relative URL but not the expected
<code class="docutils literal notranslate"><span class="pre">'/%2Fexample.com/'</span></code>.</p>
</div>
<p>It’s good practice to use <code class="docutils literal notranslate"><span class="pre">get_absolute_url()</span></code> in templates, instead of
hard-coding your objects’ URLs. For example, this template code is bad:</p>
<div class="highlight-html+django notranslate"><div class="highlight"><pre><span></span><span class="c"><!-- BAD template code. Avoid! --></span>
<span class="p"><</span><span class="nt">a</span> <span class="na">href</span><span class="o">=</span><span class="s">"/people/</span><span class="cp">{{</span> <span class="nv">object.id</span> <span class="cp">}}</span><span class="s">/"</span><span class="p">></span><span class="cp">{{</span> <span class="nv">object.name</span> <span class="cp">}}</span><span class="p"></</span><span class="nt">a</span><span class="p">></span>
</pre></div>
</div>
<p>This template code is much better:</p>
<div class="highlight-html+django notranslate"><div class="highlight"><pre><span></span><span class="p"><</span><span class="nt">a</span> <span class="na">href</span><span class="o">=</span><span class="s">"</span><span class="cp">{{</span> <span class="nv">object.get_absolute_url</span> <span class="cp">}}</span><span class="s">"</span><span class="p">></span><span class="cp">{{</span> <span class="nv">object.name</span> <span class="cp">}}</span><span class="p"></</span><span class="nt">a</span><span class="p">></span>
</pre></div>
</div>
<p>The logic here is that if you change the URL structure of your objects, even
for something simple such as correcting a spelling error, you don’t want to
have to track down every place that the URL might be created. Specify it once,
in <code class="docutils literal notranslate"><span class="pre">get_absolute_url()</span></code> and have all your other code call that one place.</p>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p>The string you return from <code class="docutils literal notranslate"><span class="pre">get_absolute_url()</span></code> <strong>must</strong> contain only
ASCII characters (required by the URI specification, <span class="target" id="index-0"></span><a class="rfc reference external" href="https://tools.ietf.org/html/rfc2396.html"><strong>RFC 2396</strong></a>) and be
URL-encoded, if necessary.</p>
<p class="last">Code and templates calling <code class="docutils literal notranslate"><span class="pre">get_absolute_url()</span></code> should be able to use the
result directly without any further processing. You may wish to use the
<code class="docutils literal notranslate"><span class="pre">django.utils.encoding.iri_to_uri()</span></code> function to help with this if you
are using unicode strings containing characters outside the ASCII range at
all.</p>
</div>
</div>
</div>
<div class="section" id="s-extra-instance-methods">
<span id="extra-instance-methods"></span><h2>Extra instance methods<a class="headerlink" href="#extra-instance-methods" title="Permalink to this headline">¶</a></h2>
<p>In addition to <a class="reference internal" href="#django.db.models.Model.save" title="django.db.models.Model.save"><code class="xref py py-meth docutils literal notranslate"><span class="pre">save()</span></code></a>, <a class="reference internal" href="#django.db.models.Model.delete" title="django.db.models.Model.delete"><code class="xref py py-meth docutils literal notranslate"><span class="pre">delete()</span></code></a>, a model object
might have some of the following methods:</p>
<dl class="method">
<dt id="django.db.models.Model.get_FOO_display">
<code class="descclassname">Model.</code><code class="descname">get_FOO_display</code>()<a class="headerlink" href="#django.db.models.Model.get_FOO_display" title="Permalink to this definition">¶</a></dt>
<dd></dd></dl>
<p>For every field that has <a class="reference internal" href="fields.html#django.db.models.Field.choices" title="django.db.models.Field.choices"><code class="xref py py-attr docutils literal notranslate"><span class="pre">choices</span></code></a> set, the
object will have a <code class="docutils literal notranslate"><span class="pre">get_FOO_display()</span></code> method, where <code class="docutils literal notranslate"><span class="pre">FOO</span></code> is the name of
the field. This method returns the “human-readable” value of the field.</p>
<p>For example:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">django.db</span> <span class="k">import</span> <span class="n">models</span>
<span class="k">class</span> <span class="nc">Person</span><span class="p">(</span><span class="n">models</span><span class="o">.</span><span class="n">Model</span><span class="p">):</span>
<span class="n">SHIRT_SIZES</span> <span class="o">=</span> <span class="p">(</span>
<span class="p">(</span><span class="s1">'S'</span><span class="p">,</span> <span class="s1">'Small'</span><span class="p">),</span>
<span class="p">(</span><span class="s1">'M'</span><span class="p">,</span> <span class="s1">'Medium'</span><span class="p">),</span>
<span class="p">(</span><span class="s1">'L'</span><span class="p">,</span> <span class="s1">'Large'</span><span class="p">),</span>
<span class="p">)</span>
<span class="n">name</span> <span class="o">=</span> <span class="n">models</span><span class="o">.</span><span class="n">CharField</span><span class="p">(</span><span class="n">max_length</span><span class="o">=</span><span class="mi">60</span><span class="p">)</span>
<span class="n">shirt_size</span> <span class="o">=</span> <span class="n">models</span><span class="o">.</span><span class="n">CharField</span><span class="p">(</span><span class="n">max_length</span><span class="o">=</span><span class="mi">2</span><span class="p">,</span> <span class="n">choices</span><span class="o">=</span><span class="n">SHIRT_SIZES</span><span class="p">)</span>
</pre></div>
</div>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">p</span> <span class="o">=</span> <span class="n">Person</span><span class="p">(</span><span class="n">name</span><span class="o">=</span><span class="s2">"Fred Flintstone"</span><span class="p">,</span> <span class="n">shirt_size</span><span class="o">=</span><span class="s2">"L"</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">p</span><span class="o">.</span><span class="n">save</span><span class="p">()</span>
<span class="gp">>>> </span><span class="n">p</span><span class="o">.</span><span class="n">shirt_size</span>
<span class="go">'L'</span>
<span class="gp">>>> </span><span class="n">p</span><span class="o">.</span><span class="n">get_shirt_size_display</span><span class="p">()</span>
<span class="go">'Large'</span>
</pre></div>
</div>
<dl class="method">
<dt id="django.db.models.Model.get_next_by_FOO">
<code class="descclassname">Model.</code><code class="descname">get_next_by_FOO</code>(<em>**kwargs</em>)<a class="headerlink" href="#django.db.models.Model.get_next_by_FOO" title="Permalink to this definition">¶</a></dt>
<dd></dd></dl>
<dl class="method">
<dt id="django.db.models.Model.get_previous_by_FOO">
<code class="descclassname">Model.</code><code class="descname">get_previous_by_FOO</code>(<em>**kwargs</em>)<a class="headerlink" href="#django.db.models.Model.get_previous_by_FOO" title="Permalink to this definition">¶</a></dt>
<dd></dd></dl>
<p>For every <a class="reference internal" href="fields.html#django.db.models.DateField" title="django.db.models.DateField"><code class="xref py py-class docutils literal notranslate"><span class="pre">DateField</span></code></a> and
<a class="reference internal" href="fields.html#django.db.models.DateTimeField" title="django.db.models.DateTimeField"><code class="xref py py-class docutils literal notranslate"><span class="pre">DateTimeField</span></code></a> that does not have <a class="reference internal" href="fields.html#django.db.models.Field.null" title="django.db.models.Field.null"><code class="xref py py-attr docutils literal notranslate"><span class="pre">null=True</span></code></a>, the object will have <code class="docutils literal notranslate"><span class="pre">get_next_by_FOO()</span></code> and
<code class="docutils literal notranslate"><span class="pre">get_previous_by_FOO()</span></code> methods, where <code class="docutils literal notranslate"><span class="pre">FOO</span></code> is the name of the field. This
returns the next and previous object with respect to the date field, raising
a <a class="reference internal" href="#django.db.models.Model.DoesNotExist" title="django.db.models.Model.DoesNotExist"><code class="xref py py-exc docutils literal notranslate"><span class="pre">DoesNotExist</span></code></a> exception when appropriate.</p>
<p>Both of these methods will perform their queries using the default
manager for the model. If you need to emulate filtering used by a
custom manager, or want to perform one-off custom filtering, both
methods also accept optional keyword arguments, which should be in the
format described in <a class="reference internal" href="querysets.html#field-lookups"><span class="std std-ref">Field lookups</span></a>.</p>
<p>Note that in the case of identical date values, these methods will use the
primary key as a tie-breaker. This guarantees that no records are skipped or
duplicated. That also means you cannot use those methods on unsaved objects.</p>
</div>
<div class="section" id="s-other-attributes">
<span id="other-attributes"></span><h2>Other attributes<a class="headerlink" href="#other-attributes" title="Permalink to this headline">¶</a></h2>
<div class="section" id="s-doesnotexist">
<span id="doesnotexist"></span><h3><code class="docutils literal notranslate"><span class="pre">DoesNotExist</span></code><a class="headerlink" href="#doesnotexist" title="Permalink to this headline">¶</a></h3>
<dl class="exception">
<dt id="django.db.models.Model.DoesNotExist">
<em class="property">exception </em><code class="descclassname">Model.</code><code class="descname">DoesNotExist</code><a class="headerlink" href="#django.db.models.Model.DoesNotExist" title="Permalink to this definition">¶</a></dt>
<dd><p>This exception is raised by the ORM in a couple places, for example by
<a class="reference internal" href="querysets.html#django.db.models.query.QuerySet.get" title="django.db.models.query.QuerySet.get"><code class="xref py py-meth docutils literal notranslate"><span class="pre">QuerySet.get()</span></code></a> when an object
is not found for the given query parameters.</p>
<p>Django provides a <code class="docutils literal notranslate"><span class="pre">DoesNotExist</span></code> exception as an attribute of each model
class to identify the class of object that could not be found and to allow
you to catch a particular model class with <code class="docutils literal notranslate"><span class="pre">try/except</span></code>. The exception is
a subclass of <a class="reference internal" href="../exceptions.html#django.core.exceptions.ObjectDoesNotExist" title="django.core.exceptions.ObjectDoesNotExist"><code class="xref py py-exc docutils literal notranslate"><span class="pre">django.core.exceptions.ObjectDoesNotExist</span></code></a>.</p>
</dd></dl>
</div>
</div>
</div>
</div>
</div>
</div>
<div class="yui-b" id="sidebar">
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<div class="sphinxsidebarwrapper">
<h3><a href="../../contents.html">Table of Contents</a></h3>
<ul>
<li><a class="reference internal" href="#">Model instance reference</a><ul>
<li><a class="reference internal" href="#creating-objects">Creating objects</a><ul>
<li><a class="reference internal" href="#customizing-model-loading">Customizing model loading</a></li>
</ul>
</li>
<li><a class="reference internal" href="#refreshing-objects-from-database">Refreshing objects from database</a></li>
<li><a class="reference internal" href="#validating-objects">Validating objects</a></li>
<li><a class="reference internal" href="#saving-objects">Saving objects</a><ul>
<li><a class="reference internal" href="#auto-incrementing-primary-keys">Auto-incrementing primary keys</a><ul>
<li><a class="reference internal" href="#the-pk-property">The <code class="docutils literal notranslate"><span class="pre">pk</span></code> property</a></li>
<li><a class="reference internal" href="#explicitly-specifying-auto-primary-key-values">Explicitly specifying auto-primary-key values</a></li>
</ul>
</li>
<li><a class="reference internal" href="#what-happens-when-you-save">What happens when you save?</a></li>
<li><a class="reference internal" href="#how-django-knows-to-update-vs-insert">How Django knows to UPDATE vs. INSERT</a><ul>
<li><a class="reference internal" href="#forcing-an-insert-or-update">Forcing an INSERT or UPDATE</a></li>
</ul>
</li>
<li><a class="reference internal" href="#updating-attributes-based-on-existing-fields">Updating attributes based on existing fields</a></li>
<li><a class="reference internal" href="#specifying-which-fields-to-save">Specifying which fields to save</a></li>
</ul>
</li>
<li><a class="reference internal" href="#deleting-objects">Deleting objects</a></li>
<li><a class="reference internal" href="#pickling-objects">Pickling objects</a></li>
<li><a class="reference internal" href="#other-model-instance-methods">Other model instance methods</a><ul>
<li><a class="reference internal" href="#str"><code class="docutils literal notranslate"><span class="pre">__str__()</span></code></a></li>
<li><a class="reference internal" href="#eq"><code class="docutils literal notranslate"><span class="pre">__eq__()</span></code></a></li>
<li><a class="reference internal" href="#hash"><code class="docutils literal notranslate"><span class="pre">__hash__()</span></code></a></li>
<li><a class="reference internal" href="#get-absolute-url"><code class="docutils literal notranslate"><span class="pre">get_absolute_url()</span></code></a></li>
</ul>
</li>
<li><a class="reference internal" href="#extra-instance-methods">Extra instance methods</a></li>
<li><a class="reference internal" href="#other-attributes">Other attributes</a><ul>
<li><a class="reference internal" href="#doesnotexist"><code class="docutils literal notranslate"><span class="pre">DoesNotExist</span></code></a></li>
</ul>
</li>
</ul>
</li>
</ul>
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