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<div class="yui-g" id="ref-models-relations">
<div class="section" id="s-related-objects-reference">
<span id="related-objects-reference"></span><h1>Related objects reference<a class="headerlink" href="#related-objects-reference" title="Permalink to this headline">¶</a></h1>
<dl class="class">
<dt id="django.db.models.fields.related.RelatedManager">
<em class="property">class </em><tt class="descname">RelatedManager</tt><a class="headerlink" href="#django.db.models.fields.related.RelatedManager" title="Permalink to this definition">¶</a></dt>
<dd><p>A “related manager” is a on managers used in a one-to-many or many-to-many
related context. This happens in two cases:</p>
<ul>
<li><p class="first">The “other side” of a <tt class="docutils literal"><span class="pre">ForeignKey</span></tt> relation. That is:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="k">class</span> <span class="nc">Reporter</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">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="n">reporter</span> <span class="o">=</span> <span class="n">models</span><span class="o">.</span><span class="n">ForeignKey</span><span class="p">(</span><span class="n">Reporter</span><span class="p">)</span>
</pre></div>
</div>
<p>In the above example, the methods below will be available on
the manager <tt class="docutils literal"><span class="pre">reporter.article_set</span></tt>.</p>
</li>
<li><p class="first">Both sides of a <tt class="docutils literal"><span class="pre">ManyToManyField</span></tt> relation:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="k">class</span> <span class="nc">Topping</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">class</span> <span class="nc">Pizza</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">toppings</span> <span class="o">=</span> <span class="n">models</span><span class="o">.</span><span class="n">ManyToManyField</span><span class="p">(</span><span class="n">Topping</span><span class="p">)</span>
</pre></div>
</div>
<p>In this example, the methods below will be available both on
<tt class="docutils literal"><span class="pre">topping.pizza_set</span></tt> and on <tt class="docutils literal"><span class="pre">pizza.toppings</span></tt>.</p>
</li>
</ul>
<p>These related managers have some extra methods:</p>
<dl class="method">
<dt id="django.db.models.fields.related.RelatedManager.add">
<tt class="descname">add</tt>(<em>obj1</em><span class="optional">[</span>, <em>obj2</em>, <em>...</em><span class="optional">]</span>)<a class="headerlink" href="#django.db.models.fields.related.RelatedManager.add" title="Permalink to this definition">¶</a></dt>
<dd><p>Adds the specified model objects to the related object set.</p>
<p>Example:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">b</span> <span class="o">=</span> <span class="n">Blog</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="nb">id</span><span class="o">=</span><span class="mi">1</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">e</span> <span class="o">=</span> <span class="n">Entry</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="nb">id</span><span class="o">=</span><span class="mi">234</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">b</span><span class="o">.</span><span class="n">entry_set</span><span class="o">.</span><span class="n">add</span><span class="p">(</span><span class="n">e</span><span class="p">)</span> <span class="c"># Associates Entry e with Blog b.</span>
</pre></div>
</div>
</dd></dl>
<dl class="method">
<dt id="django.db.models.fields.related.RelatedManager.create">
<tt class="descname">create</tt>(<em>**kwargs</em>)<a class="headerlink" href="#django.db.models.fields.related.RelatedManager.create" title="Permalink to this definition">¶</a></dt>
<dd><p>Creates a new object, saves it and puts it in the related object set.
Returns the newly created object:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">b</span> <span class="o">=</span> <span class="n">Blog</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="nb">id</span><span class="o">=</span><span class="mi">1</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">e</span> <span class="o">=</span> <span class="n">b</span><span class="o">.</span><span class="n">entry_set</span><span class="o">.</span><span class="n">create</span><span class="p">(</span>
<span class="gp">... </span> <span class="n">headline</span><span class="o">=</span><span class="s">'Hello'</span><span class="p">,</span>
<span class="gp">... </span> <span class="n">body_text</span><span class="o">=</span><span class="s">'Hi'</span><span class="p">,</span>
<span class="gp">... </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="p">(</span><span class="mi">2005</span><span class="p">,</span> <span class="mi">1</span><span class="p">,</span> <span class="mi">1</span><span class="p">)</span>
<span class="gp">... </span><span class="p">)</span>
<span class="go"># No need to call e.save() at this point -- it's already been saved.</span>
</pre></div>
</div>
<p>This is equivalent to (but much simpler than):</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">b</span> <span class="o">=</span> <span class="n">Blog</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="nb">id</span><span class="o">=</span><span class="mi">1</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">e</span> <span class="o">=</span> <span class="n">Entry</span><span class="p">(</span>
<span class="go">.... blog=b,</span>
<span class="go">.... headline='Hello',</span>
<span class="go">.... body_text='Hi',</span>
<span class="go">.... pub_date=datetime.date(2005, 1, 1)</span>
<span class="go">.... )</span>
<span class="gp">>>> </span><span class="n">e</span><span class="o">.</span><span class="n">save</span><span class="p">(</span><span class="n">force_insert</span><span class="o">=</span><span class="bp">True</span><span class="p">)</span>
</pre></div>
</div>
<p>Note that there's no need to specify the keyword argument of the model
that defines the relationship. In the above example, we don't pass the
parameter <tt class="docutils literal"><span class="pre">blog</span></tt> to <tt class="docutils literal"><span class="pre">create()</span></tt>. Django figures out that the new
<tt class="docutils literal"><span class="pre">Entry</span></tt> object's <tt class="docutils literal"><span class="pre">blog</span></tt> field should be set to <tt class="docutils literal"><span class="pre">b</span></tt>.</p>
</dd></dl>
<dl class="method">
<dt id="django.db.models.fields.related.RelatedManager.remove">
<tt class="descname">remove</tt>(<em>obj1</em><span class="optional">[</span>, <em>obj2</em>, <em>...</em><span class="optional">]</span>)<a class="headerlink" href="#django.db.models.fields.related.RelatedManager.remove" title="Permalink to this definition">¶</a></dt>
<dd><p>Removes the specified model objects from the related object set:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">b</span> <span class="o">=</span> <span class="n">Blog</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="nb">id</span><span class="o">=</span><span class="mi">1</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">e</span> <span class="o">=</span> <span class="n">Entry</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="nb">id</span><span class="o">=</span><span class="mi">234</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">b</span><span class="o">.</span><span class="n">entry_set</span><span class="o">.</span><span class="n">remove</span><span class="p">(</span><span class="n">e</span><span class="p">)</span> <span class="c"># Disassociates Entry e from Blog b.</span>
</pre></div>
</div>
<p>In order to prevent database inconsistency, this method only exists on
<tt class="docutils literal"><span class="pre">ForeignKey</span></tt> objects where <tt class="docutils literal"><span class="pre">null=True</span></tt>. If the related field can't
be set to <tt class="xref docutils literal"><span class="pre">None</span></tt> (<tt class="docutils literal"><span class="pre">NULL</span></tt>), then an object can't be removed from a
relation without being added to another. In the above example, removing
<tt class="docutils literal"><span class="pre">e</span></tt> from <tt class="docutils literal"><span class="pre">b.entry_set()</span></tt> is equivalent to doing <tt class="docutils literal"><span class="pre">e.blog</span> <span class="pre">=</span> <span class="pre">None</span></tt>,
and because the <tt class="docutils literal"><span class="pre">blog</span></tt> <tt class="docutils literal"><span class="pre">ForeignKey</span></tt> doesn't have <tt class="docutils literal"><span class="pre">null=True</span></tt>, this
is invalid.</p>
</dd></dl>
<dl class="method">
<dt id="django.db.models.fields.related.RelatedManager.clear">
<tt class="descname">clear</tt>()<a class="headerlink" href="#django.db.models.fields.related.RelatedManager.clear" title="Permalink to this definition">¶</a></dt>
<dd><p>Removes all objects from the related object set:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">b</span> <span class="o">=</span> <span class="n">Blog</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="nb">id</span><span class="o">=</span><span class="mi">1</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">b</span><span class="o">.</span><span class="n">entry_set</span><span class="o">.</span><span class="n">clear</span><span class="p">()</span>
</pre></div>
</div>
<p>Note this doesn't delete the related objects -- it just disassociates
them.</p>
<p>Just like <tt class="docutils literal"><span class="pre">remove()</span></tt>, <tt class="docutils literal"><span class="pre">clear()</span></tt> is only available on
<tt class="docutils literal"><span class="pre">ForeignKey</span></tt>s where <tt class="docutils literal"><span class="pre">null=True</span></tt>.</p>
</dd></dl>
</dd></dl>
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