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SQLAlchemy 1.2 Documentation
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Release: <span class="version-num">1.2.19</span>
| Release Date: April 15, 2019
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<h1>SQLAlchemy 1.2 Documentation</h1>
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<h3><a href="../index.html">SQLAlchemy 1.2 Documentation</a></h3>
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<h3>
<a href="index.html" title="SQLAlchemy ORM">SQLAlchemy ORM</a>
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<li><span class="link-container"><a class="reference external" href="tutorial.html">Object Relational Tutorial</a></span></li>
<li><span class="link-container"><a class="reference external" href="mapper_config.html">Mapper Configuration</a></span></li>
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<li class="selected"><span class="link-container"><strong>Loading Columns</strong><a class="paramlink headerlink reference internal" href="#">¶</a></span><ul>
<li><span class="link-container"><a class="reference external" href="#deferred-column-loading">Deferred Column Loading</a></span><ul>
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<li><span class="link-container"><a class="reference external" href="#deferred-loading-with-multiple-entities">Deferred Loading with Multiple Entities</a></span></li>
<li><span class="link-container"><a class="reference external" href="#column-deferral-api">Column Deferral API</a></span></li>
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<span class="target" id="module-sqlalchemy.orm"></span><div class="section" id="loading-columns">
<h1>Loading Columns<a class="headerlink" href="#loading-columns" title="Permalink to this headline">¶</a></h1>
<p>This section presents additional options regarding the loading of columns.</p>
<div class="section" id="deferred-column-loading">
<span id="deferred"></span><h2>Deferred Column Loading<a class="headerlink" href="#deferred-column-loading" title="Permalink to this headline">¶</a></h2>
<p>This feature allows particular columns of a table be loaded only
upon direct access, instead of when the entity is queried using
<a class="reference internal" href="query.html#sqlalchemy.orm.query.Query" title="sqlalchemy.orm.query.Query"><code class="xref py py-class docutils literal notranslate"><span class="pre">Query</span></code></a>. This feature is useful when one wants to avoid
loading a large text or binary field into memory when it’s not needed.
Individual columns can be lazy loaded by themselves or placed into groups that
lazy-load together, using the <a class="reference internal" href="#sqlalchemy.orm.deferred" title="sqlalchemy.orm.deferred"><code class="xref py py-func docutils literal notranslate"><span class="pre">orm.deferred()</span></code></a> function to
mark them as “deferred”. In the example below, we define a mapping that will load each of
<code class="docutils literal notranslate"><span class="pre">.excerpt</span></code> and <code class="docutils literal notranslate"><span class="pre">.photo</span></code> in separate, individual-row SELECT statements when each
attribute is first referenced on the individual object instance:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">sqlalchemy.orm</span> <span class="k">import</span> <span class="n">deferred</span>
<span class="kn">from</span> <span class="nn">sqlalchemy</span> <span class="k">import</span> <span class="n">Integer</span><span class="p">,</span> <span class="n">String</span><span class="p">,</span> <span class="n">Text</span><span class="p">,</span> <span class="n">Binary</span><span class="p">,</span> <span class="n">Column</span>
<span class="k">class</span> <span class="nc">Book</span><span class="p">(</span><span class="n">Base</span><span class="p">):</span>
<span class="n">__tablename__</span> <span class="o">=</span> <span class="s1">'book'</span>
<span class="n">book_id</span> <span class="o">=</span> <span class="n">Column</span><span class="p">(</span><span class="n">Integer</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="n">title</span> <span class="o">=</span> <span class="n">Column</span><span class="p">(</span><span class="n">String</span><span class="p">(</span><span class="mi">200</span><span class="p">),</span> <span class="n">nullable</span><span class="o">=</span><span class="kc">False</span><span class="p">)</span>
<span class="n">summary</span> <span class="o">=</span> <span class="n">Column</span><span class="p">(</span><span class="n">String</span><span class="p">(</span><span class="mi">2000</span><span class="p">))</span>
<span class="n">excerpt</span> <span class="o">=</span> <span class="n">deferred</span><span class="p">(</span><span class="n">Column</span><span class="p">(</span><span class="n">Text</span><span class="p">))</span>
<span class="n">photo</span> <span class="o">=</span> <span class="n">deferred</span><span class="p">(</span><span class="n">Column</span><span class="p">(</span><span class="n">Binary</span><span class="p">))</span></pre></div>
</div>
<p>Classical mappings as always place the usage of <a class="reference internal" href="#sqlalchemy.orm.deferred" title="sqlalchemy.orm.deferred"><code class="xref py py-func docutils literal notranslate"><span class="pre">orm.deferred()</span></code></a> in the
<code class="docutils literal notranslate"><span class="pre">properties</span></code> dictionary against the table-bound <a class="reference internal" href="../core/metadata.html#sqlalchemy.schema.Column" title="sqlalchemy.schema.Column"><code class="xref py py-class docutils literal notranslate"><span class="pre">Column</span></code></a>:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">mapper</span><span class="p">(</span><span class="n">Book</span><span class="p">,</span> <span class="n">book_table</span><span class="p">,</span> <span class="n">properties</span><span class="o">=</span><span class="p">{</span>
<span class="s1">'photo'</span><span class="p">:</span><span class="n">deferred</span><span class="p">(</span><span class="n">book_table</span><span class="o">.</span><span class="n">c</span><span class="o">.</span><span class="n">photo</span><span class="p">)</span>
<span class="p">})</span></pre></div>
</div>
<p>Deferred columns can be associated with a “group” name, so that they load
together when any of them are first accessed. The example below defines a
mapping with a <code class="docutils literal notranslate"><span class="pre">photos</span></code> deferred group. When one <code class="docutils literal notranslate"><span class="pre">.photo</span></code> is accessed, all three
photos will be loaded in one SELECT statement. The <code class="docutils literal notranslate"><span class="pre">.excerpt</span></code> will be loaded
separately when it is accessed:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">Book</span><span class="p">(</span><span class="n">Base</span><span class="p">):</span>
<span class="n">__tablename__</span> <span class="o">=</span> <span class="s1">'book'</span>
<span class="n">book_id</span> <span class="o">=</span> <span class="n">Column</span><span class="p">(</span><span class="n">Integer</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="n">title</span> <span class="o">=</span> <span class="n">Column</span><span class="p">(</span><span class="n">String</span><span class="p">(</span><span class="mi">200</span><span class="p">),</span> <span class="n">nullable</span><span class="o">=</span><span class="kc">False</span><span class="p">)</span>
<span class="n">summary</span> <span class="o">=</span> <span class="n">Column</span><span class="p">(</span><span class="n">String</span><span class="p">(</span><span class="mi">2000</span><span class="p">))</span>
<span class="n">excerpt</span> <span class="o">=</span> <span class="n">deferred</span><span class="p">(</span><span class="n">Column</span><span class="p">(</span><span class="n">Text</span><span class="p">))</span>
<span class="n">photo1</span> <span class="o">=</span> <span class="n">deferred</span><span class="p">(</span><span class="n">Column</span><span class="p">(</span><span class="n">Binary</span><span class="p">),</span> <span class="n">group</span><span class="o">=</span><span class="s1">'photos'</span><span class="p">)</span>
<span class="n">photo2</span> <span class="o">=</span> <span class="n">deferred</span><span class="p">(</span><span class="n">Column</span><span class="p">(</span><span class="n">Binary</span><span class="p">),</span> <span class="n">group</span><span class="o">=</span><span class="s1">'photos'</span><span class="p">)</span>
<span class="n">photo3</span> <span class="o">=</span> <span class="n">deferred</span><span class="p">(</span><span class="n">Column</span><span class="p">(</span><span class="n">Binary</span><span class="p">),</span> <span class="n">group</span><span class="o">=</span><span class="s1">'photos'</span><span class="p">)</span></pre></div>
</div>
<p>You can defer or undefer columns at the <a class="reference internal" href="query.html#sqlalchemy.orm.query.Query" title="sqlalchemy.orm.query.Query"><code class="xref py py-class docutils literal notranslate"><span class="pre">Query</span></code></a>
level using options, including <a class="reference internal" href="#sqlalchemy.orm.defer" title="sqlalchemy.orm.defer"><code class="xref py py-func docutils literal notranslate"><span class="pre">orm.defer()</span></code></a> and <a class="reference internal" href="#sqlalchemy.orm.undefer" title="sqlalchemy.orm.undefer"><code class="xref py py-func docutils literal notranslate"><span class="pre">orm.undefer()</span></code></a>:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">sqlalchemy.orm</span> <span class="k">import</span> <span class="n">defer</span><span class="p">,</span> <span class="n">undefer</span>
<span class="n">query</span> <span class="o">=</span> <span class="n">session</span><span class="o">.</span><span class="n">query</span><span class="p">(</span><span class="n">Book</span><span class="p">)</span>
<span class="n">query</span> <span class="o">=</span> <span class="n">query</span><span class="o">.</span><span class="n">options</span><span class="p">(</span><span class="n">defer</span><span class="p">(</span><span class="s1">'summary'</span><span class="p">))</span>
<span class="n">query</span> <span class="o">=</span> <span class="n">query</span><span class="o">.</span><span class="n">options</span><span class="p">(</span><span class="n">undefer</span><span class="p">(</span><span class="s1">'excerpt'</span><span class="p">))</span>
<span class="n">query</span><span class="o">.</span><span class="n">all</span><span class="p">()</span></pre></div>
</div>
<p><a class="reference internal" href="#sqlalchemy.orm.deferred" title="sqlalchemy.orm.deferred"><code class="xref py py-func docutils literal notranslate"><span class="pre">orm.deferred()</span></code></a> attributes which are marked with a “group” can be undeferred
using <a class="reference internal" href="#sqlalchemy.orm.undefer_group" title="sqlalchemy.orm.undefer_group"><code class="xref py py-func docutils literal notranslate"><span class="pre">orm.undefer_group()</span></code></a>, sending in the group name:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">sqlalchemy.orm</span> <span class="k">import</span> <span class="n">undefer_group</span>
<span class="n">query</span> <span class="o">=</span> <span class="n">session</span><span class="o">.</span><span class="n">query</span><span class="p">(</span><span class="n">Book</span><span class="p">)</span>
<span class="n">query</span><span class="o">.</span><span class="n">options</span><span class="p">(</span><span class="n">undefer_group</span><span class="p">(</span><span class="s1">'photos'</span><span class="p">))</span><span class="o">.</span><span class="n">all</span><span class="p">()</span></pre></div>
</div>
<div class="section" id="load-only-cols">
<h3>Load Only Cols<a class="headerlink" href="#load-only-cols" title="Permalink to this headline">¶</a></h3>
<p>An arbitrary set of columns can be selected as “load only” columns, which will
be loaded while deferring all other columns on a given entity, using <a class="reference internal" href="#sqlalchemy.orm.load_only" title="sqlalchemy.orm.load_only"><code class="xref py py-func docutils literal notranslate"><span class="pre">orm.load_only()</span></code></a>:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">sqlalchemy.orm</span> <span class="k">import</span> <span class="n">load_only</span>
<span class="n">session</span><span class="o">.</span><span class="n">query</span><span class="p">(</span><span class="n">Book</span><span class="p">)</span><span class="o">.</span><span class="n">options</span><span class="p">(</span><span class="n">load_only</span><span class="p">(</span><span class="s2">"summary"</span><span class="p">,</span> <span class="s2">"excerpt"</span><span class="p">))</span></pre></div>
</div>
<div class="versionadded">
<p><span class="versionmodified added">New in version 0.9.0.</span></p>
</div>
</div>
<div class="section" id="deferred-loading-with-multiple-entities">
<span id="deferred-loading-w-multiple"></span><h3>Deferred Loading with Multiple Entities<a class="headerlink" href="#deferred-loading-with-multiple-entities" title="Permalink to this headline">¶</a></h3>
<p>To specify column deferral options within a <a class="reference internal" href="query.html#sqlalchemy.orm.query.Query" title="sqlalchemy.orm.query.Query"><code class="xref py py-class docutils literal notranslate"><span class="pre">Query</span></code></a> that loads multiple types
of entity, the <a class="reference internal" href="loading_relationships.html#sqlalchemy.orm.Load" title="sqlalchemy.orm.Load"><code class="xref py py-class docutils literal notranslate"><span class="pre">Load</span></code></a> object can specify which parent entity to start with:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">sqlalchemy.orm</span> <span class="k">import</span> <span class="n">Load</span>
<span class="n">query</span> <span class="o">=</span> <span class="n">session</span><span class="o">.</span><span class="n">query</span><span class="p">(</span><span class="n">Book</span><span class="p">,</span> <span class="n">Author</span><span class="p">)</span><span class="o">.</span><span class="n">join</span><span class="p">(</span><span class="n">Book</span><span class="o">.</span><span class="n">author</span><span class="p">)</span>
<span class="n">query</span> <span class="o">=</span> <span class="n">query</span><span class="o">.</span><span class="n">options</span><span class="p">(</span>
<span class="n">Load</span><span class="p">(</span><span class="n">Book</span><span class="p">)</span><span class="o">.</span><span class="n">load_only</span><span class="p">(</span><span class="s2">"summary"</span><span class="p">,</span> <span class="s2">"excerpt"</span><span class="p">),</span>
<span class="n">Load</span><span class="p">(</span><span class="n">Author</span><span class="p">)</span><span class="o">.</span><span class="n">defer</span><span class="p">(</span><span class="s2">"bio"</span><span class="p">)</span>
<span class="p">)</span></pre></div>
</div>
<p>To specify column deferral options along the path of various relationships,
the options support chaining, where the loading style of each relationship
is specified first, then is chained to the deferral options. Such as, to load
<code class="docutils literal notranslate"><span class="pre">Book</span></code> instances, then joined-eager-load the <code class="docutils literal notranslate"><span class="pre">Author</span></code>, then apply deferral
options to the <code class="docutils literal notranslate"><span class="pre">Author</span></code> entity:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">sqlalchemy.orm</span> <span class="k">import</span> <span class="n">joinedload</span>
<span class="n">query</span> <span class="o">=</span> <span class="n">session</span><span class="o">.</span><span class="n">query</span><span class="p">(</span><span class="n">Book</span><span class="p">)</span>
<span class="n">query</span> <span class="o">=</span> <span class="n">query</span><span class="o">.</span><span class="n">options</span><span class="p">(</span>
<span class="n">joinedload</span><span class="p">(</span><span class="n">Book</span><span class="o">.</span><span class="n">author</span><span class="p">)</span><span class="o">.</span><span class="n">load_only</span><span class="p">(</span><span class="s2">"summary"</span><span class="p">,</span> <span class="s2">"excerpt"</span><span class="p">),</span>
<span class="p">)</span></pre></div>
</div>
<p>In the case where the loading style of parent relationships should be left
unchanged, use <a class="reference internal" href="loading_relationships.html#sqlalchemy.orm.defaultload" title="sqlalchemy.orm.defaultload"><code class="xref py py-func docutils literal notranslate"><span class="pre">orm.defaultload()</span></code></a>:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">sqlalchemy.orm</span> <span class="k">import</span> <span class="n">defaultload</span>
<span class="n">query</span> <span class="o">=</span> <span class="n">session</span><span class="o">.</span><span class="n">query</span><span class="p">(</span><span class="n">Book</span><span class="p">)</span>
<span class="n">query</span> <span class="o">=</span> <span class="n">query</span><span class="o">.</span><span class="n">options</span><span class="p">(</span>
<span class="n">defaultload</span><span class="p">(</span><span class="n">Book</span><span class="o">.</span><span class="n">author</span><span class="p">)</span><span class="o">.</span><span class="n">load_only</span><span class="p">(</span><span class="s2">"summary"</span><span class="p">,</span> <span class="s2">"excerpt"</span><span class="p">),</span>
<span class="p">)</span></pre></div>
</div>
<div class="versionadded">
<p><span class="versionmodified added">New in version 0.9.0: </span>support for <a class="reference internal" href="loading_relationships.html#sqlalchemy.orm.Load" title="sqlalchemy.orm.Load"><code class="xref py py-class docutils literal notranslate"><span class="pre">Load</span></code></a> and other options which
allow for better targeting of deferral options.</p>
</div>
</div>
<div class="section" id="column-deferral-api">
<h3>Column Deferral API<a class="headerlink" href="#column-deferral-api" title="Permalink to this headline">¶</a></h3>
<dl class="function">
<dt id="sqlalchemy.orm.defer">
<code class="descclassname">sqlalchemy.orm.</code><code class="descname">defer</code><span class="sig-paren">(</span><em>key</em>, <em>*addl_attrs</em><span class="sig-paren">)</span><a class="headerlink" href="#sqlalchemy.orm.defer" title="Permalink to this definition">¶</a></dt>
<dd><p>Indicate that the given column-oriented attribute should be deferred, e.g.
not loaded until accessed.</p>
<p>This function is part of the <a class="reference internal" href="loading_relationships.html#sqlalchemy.orm.Load" title="sqlalchemy.orm.Load"><code class="xref py py-class docutils literal notranslate"><span class="pre">Load</span></code></a> interface and supports
both method-chained and standalone operation.</p>
<p>e.g.:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">sqlalchemy.orm</span> <span class="k">import</span> <span class="n">defer</span>
<span class="n">session</span><span class="o">.</span><span class="n">query</span><span class="p">(</span><span class="n">MyClass</span><span class="p">)</span><span class="o">.</span><span class="n">options</span><span class="p">(</span>
<span class="n">defer</span><span class="p">(</span><span class="s2">"attribute_one"</span><span class="p">),</span>
<span class="n">defer</span><span class="p">(</span><span class="s2">"attribute_two"</span><span class="p">))</span>
<span class="n">session</span><span class="o">.</span><span class="n">query</span><span class="p">(</span><span class="n">MyClass</span><span class="p">)</span><span class="o">.</span><span class="n">options</span><span class="p">(</span>
<span class="n">defer</span><span class="p">(</span><span class="n">MyClass</span><span class="o">.</span><span class="n">attribute_one</span><span class="p">),</span>
<span class="n">defer</span><span class="p">(</span><span class="n">MyClass</span><span class="o">.</span><span class="n">attribute_two</span><span class="p">))</span></pre></div>
</div>
<p>To specify a deferred load of an attribute on a related class,
the path can be specified one token at a time, specifying the loading
style for each link along the chain. To leave the loading style
for a link unchanged, use <a class="reference internal" href="loading_relationships.html#sqlalchemy.orm.defaultload" title="sqlalchemy.orm.defaultload"><code class="xref py py-func docutils literal notranslate"><span class="pre">orm.defaultload()</span></code></a>:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">session</span><span class="o">.</span><span class="n">query</span><span class="p">(</span><span class="n">MyClass</span><span class="p">)</span><span class="o">.</span><span class="n">options</span><span class="p">(</span><span class="n">defaultload</span><span class="p">(</span><span class="s2">"someattr"</span><span class="p">)</span><span class="o">.</span><span class="n">defer</span><span class="p">(</span><span class="s2">"some_column"</span><span class="p">))</span></pre></div>
</div>
<p>A <a class="reference internal" href="loading_relationships.html#sqlalchemy.orm.Load" title="sqlalchemy.orm.Load"><code class="xref py py-class docutils literal notranslate"><span class="pre">Load</span></code></a> object that is present on a certain path can have
<a class="reference internal" href="query.html#sqlalchemy.orm.strategy_options.Load.defer" title="sqlalchemy.orm.strategy_options.Load.defer"><code class="xref py py-meth docutils literal notranslate"><span class="pre">Load.defer()</span></code></a> called multiple times, each will operate on the same
parent entity:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">session</span><span class="o">.</span><span class="n">query</span><span class="p">(</span><span class="n">MyClass</span><span class="p">)</span><span class="o">.</span><span class="n">options</span><span class="p">(</span>
<span class="n">defaultload</span><span class="p">(</span><span class="s2">"someattr"</span><span class="p">)</span><span class="o">.</span>
<span class="n">defer</span><span class="p">(</span><span class="s2">"some_column"</span><span class="p">)</span><span class="o">.</span>
<span class="n">defer</span><span class="p">(</span><span class="s2">"some_other_column"</span><span class="p">)</span><span class="o">.</span>
<span class="n">defer</span><span class="p">(</span><span class="s2">"another_column"</span><span class="p">)</span>
<span class="p">)</span></pre></div>
</div>
<dl class="field-list simple">
<dt class="field-odd">Parameters</dt>
<dd class="field-odd"><ul class="simple">
<li><p><span class="target" id="sqlalchemy.orm.defer.params.key"></span><strong>key</strong><a class="paramlink headerlink reference internal" href="#sqlalchemy.orm.defer.params.key">¶</a> – Attribute to be deferred.</p></li>
<li><p><span class="target" id="sqlalchemy.orm.defer.params.*addl_attrs"></span><strong>*addl_attrs</strong><a class="paramlink headerlink reference internal" href="#sqlalchemy.orm.defer.params.*addl_attrs">¶</a> – Deprecated; this option supports the old 0.8 style
of specifying a path as a series of attributes, which is now superseded
by the method-chained style.</p></li>
</ul>
</dd>
</dl>
<div class="admonition seealso">
<p class="admonition-title">See also</p>
<p><a class="reference internal" href="#deferred"><span class="std std-ref">Deferred Column Loading</span></a></p>
<p><a class="reference internal" href="#sqlalchemy.orm.undefer" title="sqlalchemy.orm.undefer"><code class="xref py py-func docutils literal notranslate"><span class="pre">orm.undefer()</span></code></a></p>
</div>
</dd></dl>
<dl class="function">
<dt id="sqlalchemy.orm.deferred">
<code class="descclassname">sqlalchemy.orm.</code><code class="descname">deferred</code><span class="sig-paren">(</span><em>*columns</em>, <em>**kw</em><span class="sig-paren">)</span><a class="headerlink" href="#sqlalchemy.orm.deferred" title="Permalink to this definition">¶</a></dt>
<dd><p>Indicate a column-based mapped attribute that by default will
not load unless accessed.</p>
<dl class="field-list simple">
<dt class="field-odd">Parameters</dt>
<dd class="field-odd"><ul class="simple">
<li><p><span class="target" id="sqlalchemy.orm.deferred.params.*columns"></span><strong>*columns</strong><a class="paramlink headerlink reference internal" href="#sqlalchemy.orm.deferred.params.*columns">¶</a> – columns to be mapped. This is typically a single
<a class="reference internal" href="../core/metadata.html#sqlalchemy.schema.Column" title="sqlalchemy.schema.Column"><code class="xref py py-class docutils literal notranslate"><span class="pre">Column</span></code></a> object, however a collection is supported in order
to support multiple columns mapped under the same attribute.</p></li>
<li><p><span class="target" id="sqlalchemy.orm.deferred.params.**kw"></span><strong>**kw</strong><a class="paramlink headerlink reference internal" href="#sqlalchemy.orm.deferred.params.**kw">¶</a> – additional keyword arguments passed to
<a class="reference internal" href="internals.html#sqlalchemy.orm.properties.ColumnProperty" title="sqlalchemy.orm.properties.ColumnProperty"><code class="xref py py-class docutils literal notranslate"><span class="pre">ColumnProperty</span></code></a>.</p></li>
</ul>
</dd>
</dl>
<div class="admonition seealso">
<p class="admonition-title">See also</p>
<p><a class="reference internal" href="#deferred"><span class="std std-ref">Deferred Column Loading</span></a></p>
</div>
</dd></dl>
<dl class="function">
<dt id="sqlalchemy.orm.query_expression">
<code class="descclassname">sqlalchemy.orm.</code><code class="descname">query_expression</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#sqlalchemy.orm.query_expression" title="Permalink to this definition">¶</a></dt>
<dd><p>Indicate an attribute that populates from a query-time SQL expression.</p>
<div class="versionadded">
<p><span class="versionmodified added">New in version 1.2.</span></p>
</div>
<div class="admonition seealso">
<p class="admonition-title">See also</p>
<p><span class="xref std std-ref">mapper_query_expression</span></p>
</div>
</dd></dl>
<dl class="function">
<dt id="sqlalchemy.orm.load_only">
<code class="descclassname">sqlalchemy.orm.</code><code class="descname">load_only</code><span class="sig-paren">(</span><em>*attrs</em><span class="sig-paren">)</span><a class="headerlink" href="#sqlalchemy.orm.load_only" title="Permalink to this definition">¶</a></dt>
<dd><p>Indicate that for a particular entity, only the given list
of column-based attribute names should be loaded; all others will be
deferred.</p>
<p>This function is part of the <a class="reference internal" href="loading_relationships.html#sqlalchemy.orm.Load" title="sqlalchemy.orm.Load"><code class="xref py py-class docutils literal notranslate"><span class="pre">Load</span></code></a> interface and supports
both method-chained and standalone operation.</p>
<p>Example - given a class <code class="docutils literal notranslate"><span class="pre">User</span></code>, load only the <code class="docutils literal notranslate"><span class="pre">name</span></code> and <code class="docutils literal notranslate"><span class="pre">fullname</span></code>
attributes:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">session</span><span class="o">.</span><span class="n">query</span><span class="p">(</span><span class="n">User</span><span class="p">)</span><span class="o">.</span><span class="n">options</span><span class="p">(</span><span class="n">load_only</span><span class="p">(</span><span class="s2">"name"</span><span class="p">,</span> <span class="s2">"fullname"</span><span class="p">))</span></pre></div>
</div>
<p>Example - given a relationship <code class="docutils literal notranslate"><span class="pre">User.addresses</span> <span class="pre">-></span> <span class="pre">Address</span></code>, specify
subquery loading for the <code class="docutils literal notranslate"><span class="pre">User.addresses</span></code> collection, but on each
<code class="docutils literal notranslate"><span class="pre">Address</span></code> object load only the <code class="docutils literal notranslate"><span class="pre">email_address</span></code> attribute:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">session</span><span class="o">.</span><span class="n">query</span><span class="p">(</span><span class="n">User</span><span class="p">)</span><span class="o">.</span><span class="n">options</span><span class="p">(</span>
<span class="n">subqueryload</span><span class="p">(</span><span class="s2">"addresses"</span><span class="p">)</span><span class="o">.</span><span class="n">load_only</span><span class="p">(</span><span class="s2">"email_address"</span><span class="p">)</span>
<span class="p">)</span></pre></div>
</div>
<p>For a <a class="reference internal" href="query.html#sqlalchemy.orm.query.Query" title="sqlalchemy.orm.query.Query"><code class="xref py py-class docutils literal notranslate"><span class="pre">Query</span></code></a> that has multiple entities, the lead entity can be
specifically referred to using the <a class="reference internal" href="loading_relationships.html#sqlalchemy.orm.Load" title="sqlalchemy.orm.Load"><code class="xref py py-class docutils literal notranslate"><span class="pre">Load</span></code></a> constructor:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">session</span><span class="o">.</span><span class="n">query</span><span class="p">(</span><span class="n">User</span><span class="p">,</span> <span class="n">Address</span><span class="p">)</span><span class="o">.</span><span class="n">join</span><span class="p">(</span><span class="n">User</span><span class="o">.</span><span class="n">addresses</span><span class="p">)</span><span class="o">.</span><span class="n">options</span><span class="p">(</span>
<span class="n">Load</span><span class="p">(</span><span class="n">User</span><span class="p">)</span><span class="o">.</span><span class="n">load_only</span><span class="p">(</span><span class="s2">"name"</span><span class="p">,</span> <span class="s2">"fullname"</span><span class="p">),</span>
<span class="n">Load</span><span class="p">(</span><span class="n">Address</span><span class="p">)</span><span class="o">.</span><span class="n">load_only</span><span class="p">(</span><span class="s2">"email_addres"</span><span class="p">)</span>
<span class="p">)</span></pre></div>
</div>
<div class="versionadded">
<p><span class="versionmodified added">New in version 0.9.0.</span></p>
</div>
</dd></dl>
<dl class="function">
<dt id="sqlalchemy.orm.undefer">
<code class="descclassname">sqlalchemy.orm.</code><code class="descname">undefer</code><span class="sig-paren">(</span><em>key</em>, <em>*addl_attrs</em><span class="sig-paren">)</span><a class="headerlink" href="#sqlalchemy.orm.undefer" title="Permalink to this definition">¶</a></dt>
<dd><p>Indicate that the given column-oriented attribute should be undeferred,
e.g. specified within the SELECT statement of the entity as a whole.</p>
<p>The column being undeferred is typically set up on the mapping as a
<a class="reference internal" href="#sqlalchemy.orm.deferred" title="sqlalchemy.orm.deferred"><code class="xref py py-func docutils literal notranslate"><span class="pre">deferred()</span></code></a> attribute.</p>
<p>This function is part of the <a class="reference internal" href="loading_relationships.html#sqlalchemy.orm.Load" title="sqlalchemy.orm.Load"><code class="xref py py-class docutils literal notranslate"><span class="pre">Load</span></code></a> interface and supports
both method-chained and standalone operation.</p>
<p>Examples:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="c1"># undefer two columns</span>
<span class="n">session</span><span class="o">.</span><span class="n">query</span><span class="p">(</span><span class="n">MyClass</span><span class="p">)</span><span class="o">.</span><span class="n">options</span><span class="p">(</span><span class="n">undefer</span><span class="p">(</span><span class="s2">"col1"</span><span class="p">),</span> <span class="n">undefer</span><span class="p">(</span><span class="s2">"col2"</span><span class="p">))</span>
<span class="c1"># undefer all columns specific to a single class using Load + *</span>
<span class="n">session</span><span class="o">.</span><span class="n">query</span><span class="p">(</span><span class="n">MyClass</span><span class="p">,</span> <span class="n">MyOtherClass</span><span class="p">)</span><span class="o">.</span><span class="n">options</span><span class="p">(</span>
<span class="n">Load</span><span class="p">(</span><span class="n">MyClass</span><span class="p">)</span><span class="o">.</span><span class="n">undefer</span><span class="p">(</span><span class="s2">"*"</span><span class="p">))</span></pre></div>
</div>
<dl class="field-list simple">
<dt class="field-odd">Parameters</dt>
<dd class="field-odd"><ul class="simple">
<li><p><span class="target" id="sqlalchemy.orm.undefer.params.key"></span><strong>key</strong><a class="paramlink headerlink reference internal" href="#sqlalchemy.orm.undefer.params.key">¶</a> – Attribute to be undeferred.</p></li>
<li><p><span class="target" id="sqlalchemy.orm.undefer.params.*addl_attrs"></span><strong>*addl_attrs</strong><a class="paramlink headerlink reference internal" href="#sqlalchemy.orm.undefer.params.*addl_attrs">¶</a> – Deprecated; this option supports the old 0.8 style
of specifying a path as a series of attributes, which is now superseded
by the method-chained style.</p></li>
</ul>
</dd>
</dl>
<div class="admonition seealso">
<p class="admonition-title">See also</p>
<p><a class="reference internal" href="#deferred"><span class="std std-ref">Deferred Column Loading</span></a></p>
<p><a class="reference internal" href="#sqlalchemy.orm.defer" title="sqlalchemy.orm.defer"><code class="xref py py-func docutils literal notranslate"><span class="pre">orm.defer()</span></code></a></p>
<p><a class="reference internal" href="#sqlalchemy.orm.undefer_group" title="sqlalchemy.orm.undefer_group"><code class="xref py py-func docutils literal notranslate"><span class="pre">orm.undefer_group()</span></code></a></p>
</div>
</dd></dl>
<dl class="function">
<dt id="sqlalchemy.orm.undefer_group">
<code class="descclassname">sqlalchemy.orm.</code><code class="descname">undefer_group</code><span class="sig-paren">(</span><em>name</em><span class="sig-paren">)</span><a class="headerlink" href="#sqlalchemy.orm.undefer_group" title="Permalink to this definition">¶</a></dt>
<dd><p>Indicate that columns within the given deferred group name should be
undeferred.</p>
<p>The columns being undeferred are set up on the mapping as
<a class="reference internal" href="#sqlalchemy.orm.deferred" title="sqlalchemy.orm.deferred"><code class="xref py py-func docutils literal notranslate"><span class="pre">deferred()</span></code></a> attributes and include a “group” name.</p>
<p>E.g:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">session</span><span class="o">.</span><span class="n">query</span><span class="p">(</span><span class="n">MyClass</span><span class="p">)</span><span class="o">.</span><span class="n">options</span><span class="p">(</span><span class="n">undefer_group</span><span class="p">(</span><span class="s2">"large_attrs"</span><span class="p">))</span></pre></div>
</div>
<p>To undefer a group of attributes on a related entity, the path can be
spelled out using relationship loader options, such as
<a class="reference internal" href="loading_relationships.html#sqlalchemy.orm.defaultload" title="sqlalchemy.orm.defaultload"><code class="xref py py-func docutils literal notranslate"><span class="pre">orm.defaultload()</span></code></a>:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">session</span><span class="o">.</span><span class="n">query</span><span class="p">(</span><span class="n">MyClass</span><span class="p">)</span><span class="o">.</span><span class="n">options</span><span class="p">(</span>
<span class="n">defaultload</span><span class="p">(</span><span class="s2">"someattr"</span><span class="p">)</span><span class="o">.</span><span class="n">undefer_group</span><span class="p">(</span><span class="s2">"large_attrs"</span><span class="p">))</span></pre></div>
</div>
<div class="versionchanged">
<p><span class="versionmodified changed">Changed in version 0.9.0: </span><a class="reference internal" href="#sqlalchemy.orm.undefer_group" title="sqlalchemy.orm.undefer_group"><code class="xref py py-func docutils literal notranslate"><span class="pre">orm.undefer_group()</span></code></a> is now specific to a
particular entity load path.</p>
</div>
<div class="admonition seealso">
<p class="admonition-title">See also</p>
<p><a class="reference internal" href="#deferred"><span class="std std-ref">Deferred Column Loading</span></a></p>
<p><a class="reference internal" href="#sqlalchemy.orm.defer" title="sqlalchemy.orm.defer"><code class="xref py py-func docutils literal notranslate"><span class="pre">orm.defer()</span></code></a></p>
<p><a class="reference internal" href="#sqlalchemy.orm.undefer" title="sqlalchemy.orm.undefer"><code class="xref py py-func docutils literal notranslate"><span class="pre">orm.undefer()</span></code></a></p>
</div>
</dd></dl>
<dl class="function">
<dt id="sqlalchemy.orm.with_expression">
<code class="descclassname">sqlalchemy.orm.</code><code class="descname">with_expression</code><span class="sig-paren">(</span><em>key</em>, <em>expression</em><span class="sig-paren">)</span><a class="headerlink" href="#sqlalchemy.orm.with_expression" title="Permalink to this definition">¶</a></dt>
<dd><p>Apply an ad-hoc SQL expression to a “deferred expression” attribute.</p>
<p>This option is used in conjunction with the <a class="reference internal" href="#sqlalchemy.orm.query_expression" title="sqlalchemy.orm.query_expression"><code class="xref py py-func docutils literal notranslate"><span class="pre">orm.query_expression()</span></code></a>
mapper-level construct that indicates an attribute which should be the
target of an ad-hoc SQL expression.</p>
<p>E.g.:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">sess</span><span class="o">.</span><span class="n">query</span><span class="p">(</span><span class="n">SomeClass</span><span class="p">)</span><span class="o">.</span><span class="n">options</span><span class="p">(</span>
<span class="n">with_expression</span><span class="p">(</span><span class="n">SomeClass</span><span class="o">.</span><span class="n">x_y_expr</span><span class="p">,</span> <span class="n">SomeClass</span><span class="o">.</span><span class="n">x</span> <span class="o">+</span> <span class="n">SomeClass</span><span class="o">.</span><span class="n">y</span><span class="p">)</span>
<span class="p">)</span></pre></div>
</div>
<div class="versionadded">
<p><span class="versionmodified added">New in version 1.2.</span></p>
</div>
<dl class="field-list simple">
<dt class="field-odd">Parameters</dt>
<dd class="field-odd"><ul class="simple">
<li><p><span class="target" id="sqlalchemy.orm.with_expression.params.key"></span><strong>key</strong><a class="paramlink headerlink reference internal" href="#sqlalchemy.orm.with_expression.params.key">¶</a> – Attribute to be undeferred.</p></li>
<li><p><span class="target" id="sqlalchemy.orm.with_expression.params.expr"></span><strong>expr</strong><a class="paramlink headerlink reference internal" href="#sqlalchemy.orm.with_expression.params.expr">¶</a> – SQL expression to be applied to the attribute.</p></li>
</ul>
</dd>
</dl>
<div class="admonition seealso">
<p class="admonition-title">See also</p>
<p><span class="xref std std-ref">mapper_query_expression</span></p>
</div>
</dd></dl>
</div>
</div>
<div class="section" id="column-bundles">
<span id="bundles"></span><h2>Column Bundles<a class="headerlink" href="#column-bundles" title="Permalink to this headline">¶</a></h2>
<p>The <a class="reference internal" href="query.html#sqlalchemy.orm.query.Bundle" title="sqlalchemy.orm.query.Bundle"><code class="xref py py-class docutils literal notranslate"><span class="pre">Bundle</span></code></a> may be used to query for groups of columns under one
namespace.</p>
<div class="versionadded">
<p><span class="versionmodified added">New in version 0.9.0.</span></p>
</div>
<p>The bundle allows columns to be grouped together:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">sqlalchemy.orm</span> <span class="k">import</span> <span class="n">Bundle</span>
<span class="n">bn</span> <span class="o">=</span> <span class="n">Bundle</span><span class="p">(</span><span class="s1">'mybundle'</span><span class="p">,</span> <span class="n">MyClass</span><span class="o">.</span><span class="n">data1</span><span class="p">,</span> <span class="n">MyClass</span><span class="o">.</span><span class="n">data2</span><span class="p">)</span>
<span class="k">for</span> <span class="n">row</span> <span class="ow">in</span> <span class="n">session</span><span class="o">.</span><span class="n">query</span><span class="p">(</span><span class="n">bn</span><span class="p">)</span><span class="o">.</span><span class="n">filter</span><span class="p">(</span><span class="n">bn</span><span class="o">.</span><span class="n">c</span><span class="o">.</span><span class="n">data1</span> <span class="o">==</span> <span class="s1">'d1'</span><span class="p">):</span>
<span class="nb">print</span><span class="p">(</span><span class="n">row</span><span class="o">.</span><span class="n">mybundle</span><span class="o">.</span><span class="n">data1</span><span class="p">,</span> <span class="n">row</span><span class="o">.</span><span class="n">mybundle</span><span class="o">.</span><span class="n">data2</span><span class="p">)</span></pre></div>
</div>
<p>The bundle can be subclassed to provide custom behaviors when results
are fetched. The method <a class="reference internal" href="query.html#sqlalchemy.orm.query.Bundle.create_row_processor" title="sqlalchemy.orm.query.Bundle.create_row_processor"><code class="xref py py-meth docutils literal notranslate"><span class="pre">Bundle.create_row_processor()</span></code></a> is given
the <a class="reference internal" href="query.html#sqlalchemy.orm.query.Query" title="sqlalchemy.orm.query.Query"><code class="xref py py-class docutils literal notranslate"><span class="pre">Query</span></code></a> and a set of “row processor” functions at query execution
time; these processor functions when given a result row will return the
individual attribute value, which can then be adapted into any kind of
return data structure. Below illustrates replacing the usual <a class="reference internal" href="query.html#sqlalchemy.util.KeyedTuple" title="sqlalchemy.util.KeyedTuple"><code class="xref py py-class docutils literal notranslate"><span class="pre">KeyedTuple</span></code></a>
return structure with a straight Python dictionary:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">sqlalchemy.orm</span> <span class="k">import</span> <span class="n">Bundle</span>
<span class="k">class</span> <span class="nc">DictBundle</span><span class="p">(</span><span class="n">Bundle</span><span class="p">):</span>
<span class="k">def</span> <span class="nf">create_row_processor</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">query</span><span class="p">,</span> <span class="n">procs</span><span class="p">,</span> <span class="n">labels</span><span class="p">):</span>
<span class="sd">"""Override create_row_processor to return values as dictionaries"""</span>
<span class="k">def</span> <span class="nf">proc</span><span class="p">(</span><span class="n">row</span><span class="p">):</span>
<span class="k">return</span> <span class="nb">dict</span><span class="p">(</span>
<span class="nb">zip</span><span class="p">(</span><span class="n">labels</span><span class="p">,</span> <span class="p">(</span><span class="n">proc</span><span class="p">(</span><span class="n">row</span><span class="p">)</span> <span class="k">for</span> <span class="n">proc</span> <span class="ow">in</span> <span class="n">procs</span><span class="p">))</span>
<span class="p">)</span>
<span class="k">return</span> <span class="n">proc</span></pre></div>
</div>
<div class="versionchanged">
<p><span class="versionmodified changed">Changed in version 1.0: </span>The <code class="docutils literal notranslate"><span class="pre">proc()</span></code> callable passed to the <code class="docutils literal notranslate"><span class="pre">create_row_processor()</span></code>
method of custom <a class="reference internal" href="query.html#sqlalchemy.orm.query.Bundle" title="sqlalchemy.orm.query.Bundle"><code class="xref py py-class docutils literal notranslate"><span class="pre">Bundle</span></code></a> classes now accepts only a single
“row” argument.</p>
</div>
<p>A result from the above bundle will return dictionary values:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">bn</span> <span class="o">=</span> <span class="n">DictBundle</span><span class="p">(</span><span class="s1">'mybundle'</span><span class="p">,</span> <span class="n">MyClass</span><span class="o">.</span><span class="n">data1</span><span class="p">,</span> <span class="n">MyClass</span><span class="o">.</span><span class="n">data2</span><span class="p">)</span>
<span class="k">for</span> <span class="n">row</span> <span class="ow">in</span> <span class="n">session</span><span class="o">.</span><span class="n">query</span><span class="p">(</span><span class="n">bn</span><span class="p">)</span><span class="o">.</span><span class="n">filter</span><span class="p">(</span><span class="n">bn</span><span class="o">.</span><span class="n">c</span><span class="o">.</span><span class="n">data1</span> <span class="o">==</span> <span class="s1">'d1'</span><span class="p">):</span>
<span class="nb">print</span><span class="p">(</span><span class="n">row</span><span class="o">.</span><span class="n">mybundle</span><span class="p">[</span><span class="s1">'data1'</span><span class="p">],</span> <span class="n">row</span><span class="o">.</span><span class="n">mybundle</span><span class="p">[</span><span class="s1">'data2'</span><span class="p">])</span></pre></div>
</div>
<p>The <a class="reference internal" href="query.html#sqlalchemy.orm.query.Bundle" title="sqlalchemy.orm.query.Bundle"><code class="xref py py-class docutils literal notranslate"><span class="pre">Bundle</span></code></a> construct is also integrated into the behavior
of <a class="reference internal" href="composites.html#sqlalchemy.orm.composite" title="sqlalchemy.orm.composite"><code class="xref py py-func docutils literal notranslate"><span class="pre">composite()</span></code></a>, where it is used to return composite attributes as objects
when queried as individual attributes.</p>
</div>
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