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Association Proxy
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<h1>SQLAlchemy 0.6.8 Documentation</h1>
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Version: <span class="versionnum">0.6.8</span> Last Updated: 06/05/2011 13:10:26
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Association Proxy
<h2>
Association Proxy
</h2>
<ul>
<li><a class="reference internal" href="#">Association Proxy</a><ul>
<li><a class="reference internal" href="#simplifying-relationships">Simplifying Relationships</a></li>
<li><a class="reference internal" href="#simplifying-association-object-relationships">Simplifying Association Object Relationships</a></li>
<li><a class="reference internal" href="#building-complex-views">Building Complex Views</a></li>
<li><a class="reference internal" href="#api">API</a></li>
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<div class="section" id="module-sqlalchemy.ext.associationproxy">
<span id="association-proxy"></span><span id="associationproxy"></span><h1>Association Proxy<a class="headerlink" href="#module-sqlalchemy.ext.associationproxy" title="Permalink to this headline">¶</a></h1>
<p><tt class="docutils literal"><span class="pre">associationproxy</span></tt> is used to create a simplified, read/write view of a
relationship. It can be used to cherry-pick fields from a collection of
related objects or to greatly simplify access to associated objects in an
association relationship.</p>
<div class="section" id="simplifying-relationships">
<h2>Simplifying Relationships<a class="headerlink" href="#simplifying-relationships" title="Permalink to this headline">¶</a></h2>
<p>Consider this “association object” mapping:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="n">users_table</span> <span class="o">=</span> <span class="n">Table</span><span class="p">(</span><span class="s">'users'</span><span class="p">,</span> <span class="n">metadata</span><span class="p">,</span>
<span class="n">Column</span><span class="p">(</span><span class="s">'id'</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="bp">True</span><span class="p">),</span>
<span class="n">Column</span><span class="p">(</span><span class="s">'name'</span><span class="p">,</span> <span class="n">String</span><span class="p">(</span><span class="mi">64</span><span class="p">)),</span>
<span class="p">)</span>
<span class="n">keywords_table</span> <span class="o">=</span> <span class="n">Table</span><span class="p">(</span><span class="s">'keywords'</span><span class="p">,</span> <span class="n">metadata</span><span class="p">,</span>
<span class="n">Column</span><span class="p">(</span><span class="s">'id'</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="bp">True</span><span class="p">),</span>
<span class="n">Column</span><span class="p">(</span><span class="s">'keyword'</span><span class="p">,</span> <span class="n">String</span><span class="p">(</span><span class="mi">64</span><span class="p">))</span>
<span class="p">)</span>
<span class="n">userkeywords_table</span> <span class="o">=</span> <span class="n">Table</span><span class="p">(</span><span class="s">'userkeywords'</span><span class="p">,</span> <span class="n">metadata</span><span class="p">,</span>
<span class="n">Column</span><span class="p">(</span><span class="s">'user_id'</span><span class="p">,</span> <span class="n">Integer</span><span class="p">,</span> <span class="n">ForeignKey</span><span class="p">(</span><span class="s">"users.id"</span><span class="p">),</span>
<span class="n">primary_key</span><span class="o">=</span><span class="bp">True</span><span class="p">),</span>
<span class="n">Column</span><span class="p">(</span><span class="s">'keyword_id'</span><span class="p">,</span> <span class="n">Integer</span><span class="p">,</span> <span class="n">ForeignKey</span><span class="p">(</span><span class="s">"keywords.id"</span><span class="p">),</span>
<span class="n">primary_key</span><span class="o">=</span><span class="bp">True</span><span class="p">)</span>
<span class="p">)</span>
<span class="k">class</span> <span class="nc">User</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span>
<span class="k">def</span> <span class="nf">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">name</span><span class="p">):</span>
<span class="bp">self</span><span class="o">.</span><span class="n">name</span> <span class="o">=</span> <span class="n">name</span>
<span class="k">class</span> <span class="nc">Keyword</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span>
<span class="k">def</span> <span class="nf">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">keyword</span><span class="p">):</span>
<span class="bp">self</span><span class="o">.</span><span class="n">keyword</span> <span class="o">=</span> <span class="n">keyword</span>
<span class="n">mapper</span><span class="p">(</span><span class="n">User</span><span class="p">,</span> <span class="n">users_table</span><span class="p">,</span> <span class="n">properties</span><span class="o">=</span><span class="p">{</span>
<span class="s">'kw'</span><span class="p">:</span> <span class="n">relationship</span><span class="p">(</span><span class="n">Keyword</span><span class="p">,</span> <span class="n">secondary</span><span class="o">=</span><span class="n">userkeywords_table</span><span class="p">)</span>
<span class="p">})</span>
<span class="n">mapper</span><span class="p">(</span><span class="n">Keyword</span><span class="p">,</span> <span class="n">keywords_table</span><span class="p">)</span></pre></div>
</div>
<p>Above are three simple tables, modeling users, keywords and a many-to-many
relationship between the two. These <tt class="docutils literal"><span class="pre">Keyword</span></tt> objects are little more
than a container for a name, and accessing them via the relationship is
awkward:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="n">user</span> <span class="o">=</span> <span class="n">User</span><span class="p">(</span><span class="s">'jek'</span><span class="p">)</span>
<span class="n">user</span><span class="o">.</span><span class="n">kw</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="n">Keyword</span><span class="p">(</span><span class="s">'cheese inspector'</span><span class="p">))</span>
<span class="k">print</span> <span class="n">user</span><span class="o">.</span><span class="n">kw</span>
<span class="c"># [<__main__.Keyword object at 0xb791ea0c>]</span>
<span class="k">print</span> <span class="n">user</span><span class="o">.</span><span class="n">kw</span><span class="p">[</span><span class="mi">0</span><span class="p">]</span><span class="o">.</span><span class="n">keyword</span>
<span class="c"># 'cheese inspector'</span>
<span class="k">print</span> <span class="p">[</span><span class="n">keyword</span><span class="o">.</span><span class="n">keyword</span> <span class="k">for</span> <span class="n">keyword</span> <span class="ow">in</span> <span class="n">user</span><span class="o">.</span><span class="n">kw</span><span class="p">]</span>
<span class="c"># ['cheese inspector']</span></pre></div>
</div>
<p>With <tt class="docutils literal"><span class="pre">association_proxy</span></tt> you have a “view” of the relationship that contains
just the <tt class="docutils literal"><span class="pre">.keyword</span></tt> of the related objects. The proxy is a Python
property, and unlike the mapper relationship, is defined in your class:</p>
<div class="highlight-python"><pre>from sqlalchemy.ext.associationproxy import association_proxy
class User(object):
def __init__(self, name):
self.name = name
# proxy the 'keyword' attribute from the 'kw' relationship
keywords = association_proxy('kw', 'keyword')
# ...
>>> user.kw
[<__main__.Keyword object at 0xb791ea0c>]
>>> user.keywords
['cheese inspector']
>>> user.keywords.append('snack ninja')
>>> user.keywords
['cheese inspector', 'snack ninja']
>>> user.kw
[<__main__.Keyword object at 0x9272a4c>, <__main__.Keyword object at 0xb7b396ec>]</pre>
</div>
<p>The proxy is read/write. New associated objects are created on demand when
values are added to the proxy, and modifying or removing an entry through
the proxy also affects the underlying collection.</p>
<blockquote>
<div><ul class="simple">
<li>The association proxy property is backed by a mapper-defined relationship,
either a collection or scalar.</li>
<li>You can access and modify both the proxy and the backing
relationship. Changes in one are immediate in the other.</li>
<li>The proxy acts like the type of the underlying collection. A list gets a
list-like proxy, a dict a dict-like proxy, and so on.</li>
<li>Multiple proxies for the same relationship are fine.</li>
<li>Proxies are lazy, and won’t trigger a load of the backing relationship until
they are accessed.</li>
<li>The relationship is inspected to determine the type of the related objects.</li>
<li>To construct new instances, the type is called with the value being
assigned, or key and value for dicts.</li>
<li>A <tt class="docutils literal"><span class="pre">``creator``</span></tt> function can be used to create instances instead.</li>
</ul>
</div></blockquote>
<p>Above, the <tt class="docutils literal"><span class="pre">Keyword.__init__</span></tt> takes a single argument <tt class="docutils literal"><span class="pre">keyword</span></tt>, which
maps conveniently to the value being set through the proxy. A <tt class="docutils literal"><span class="pre">creator</span></tt>
function could have been used instead if more flexibility was required.</p>
<p>Because the proxies are backed by a regular relationship collection, all of the
usual hooks and patterns for using collections are still in effect. The
most convenient behavior is the automatic setting of “parent”-type
relationships on assignment. In the example above, nothing special had to
be done to associate the Keyword to the User. Simply adding it to the
collection is sufficient.</p>
</div>
<div class="section" id="simplifying-association-object-relationships">
<h2>Simplifying Association Object Relationships<a class="headerlink" href="#simplifying-association-object-relationships" title="Permalink to this headline">¶</a></h2>
<p>Association proxies are also useful for keeping <tt class="docutils literal"><span class="pre">association</span> <span class="pre">objects</span></tt> out
the way during regular use. For example, the <tt class="docutils literal"><span class="pre">userkeywords</span></tt> table
might have a bunch of auditing columns that need to get updated when changes
are made- columns that are updated but seldom, if ever, accessed in your
application. A proxy can provide a very natural access pattern for the
relationship.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">from</span> <span class="nn">sqlalchemy.ext.associationproxy</span> <span class="kn">import</span> <span class="n">association_proxy</span>
<span class="c"># users_table and keywords_table tables as above, then:</span>
<span class="k">def</span> <span class="nf">get_current_uid</span><span class="p">():</span>
<span class="sd">"""Return the uid of the current user."""</span>
<span class="k">return</span> <span class="mi">1</span> <span class="c"># hardcoded for this example</span>
<span class="n">userkeywords_table</span> <span class="o">=</span> <span class="n">Table</span><span class="p">(</span><span class="s">'userkeywords'</span><span class="p">,</span> <span class="n">metadata</span><span class="p">,</span>
<span class="n">Column</span><span class="p">(</span><span class="s">'user_id'</span><span class="p">,</span> <span class="n">Integer</span><span class="p">,</span> <span class="n">ForeignKey</span><span class="p">(</span><span class="s">"users.id"</span><span class="p">),</span> <span class="n">primary_key</span><span class="o">=</span><span class="bp">True</span><span class="p">),</span>
<span class="n">Column</span><span class="p">(</span><span class="s">'keyword_id'</span><span class="p">,</span> <span class="n">Integer</span><span class="p">,</span> <span class="n">ForeignKey</span><span class="p">(</span><span class="s">"keywords.id"</span><span class="p">),</span> <span class="n">primary_key</span><span class="o">=</span><span class="bp">True</span><span class="p">),</span>
<span class="c"># add some auditing columns</span>
<span class="n">Column</span><span class="p">(</span><span class="s">'updated_at'</span><span class="p">,</span> <span class="n">DateTime</span><span class="p">,</span> <span class="n">default</span><span class="o">=</span><span class="n">datetime</span><span class="o">.</span><span class="n">now</span><span class="p">),</span>
<span class="n">Column</span><span class="p">(</span><span class="s">'updated_by'</span><span class="p">,</span> <span class="n">Integer</span><span class="p">,</span> <span class="n">default</span><span class="o">=</span><span class="n">get_current_uid</span><span class="p">,</span> <span class="n">onupdate</span><span class="o">=</span><span class="n">get_current_uid</span><span class="p">),</span>
<span class="p">)</span>
<span class="k">def</span> <span class="nf">_create_uk_by_keyword</span><span class="p">(</span><span class="n">keyword</span><span class="p">):</span>
<span class="sd">"""A creator function."""</span>
<span class="k">return</span> <span class="n">UserKeyword</span><span class="p">(</span><span class="n">keyword</span><span class="o">=</span><span class="n">keyword</span><span class="p">)</span>
<span class="k">class</span> <span class="nc">User</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span>
<span class="k">def</span> <span class="nf">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">name</span><span class="p">):</span>
<span class="bp">self</span><span class="o">.</span><span class="n">name</span> <span class="o">=</span> <span class="n">name</span>
<span class="n">keywords</span> <span class="o">=</span> <span class="n">association_proxy</span><span class="p">(</span><span class="s">'user_keywords'</span><span class="p">,</span> <span class="s">'keyword'</span><span class="p">,</span> <span class="n">creator</span><span class="o">=</span><span class="n">_create_uk_by_keyword</span><span class="p">)</span>
<span class="k">class</span> <span class="nc">Keyword</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span>
<span class="k">def</span> <span class="nf">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">keyword</span><span class="p">):</span>
<span class="bp">self</span><span class="o">.</span><span class="n">keyword</span> <span class="o">=</span> <span class="n">keyword</span>
<span class="k">def</span> <span class="nf">__repr__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="k">return</span> <span class="s">'Keyword(</span><span class="si">%s</span><span class="s">)'</span> <span class="o">%</span> <span class="nb">repr</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">keyword</span><span class="p">)</span>
<span class="k">class</span> <span class="nc">UserKeyword</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span>
<span class="k">def</span> <span class="nf">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">user</span><span class="o">=</span><span class="bp">None</span><span class="p">,</span> <span class="n">keyword</span><span class="o">=</span><span class="bp">None</span><span class="p">):</span>
<span class="bp">self</span><span class="o">.</span><span class="n">user</span> <span class="o">=</span> <span class="n">user</span>
<span class="bp">self</span><span class="o">.</span><span class="n">keyword</span> <span class="o">=</span> <span class="n">keyword</span>
<span class="n">mapper</span><span class="p">(</span><span class="n">User</span><span class="p">,</span> <span class="n">users_table</span><span class="p">)</span>
<span class="n">mapper</span><span class="p">(</span><span class="n">Keyword</span><span class="p">,</span> <span class="n">keywords_table</span><span class="p">)</span>
<span class="n">mapper</span><span class="p">(</span><span class="n">UserKeyword</span><span class="p">,</span> <span class="n">userkeywords_table</span><span class="p">,</span> <span class="n">properties</span><span class="o">=</span><span class="p">{</span>
<span class="s">'user'</span><span class="p">:</span> <span class="n">relationship</span><span class="p">(</span><span class="n">User</span><span class="p">,</span> <span class="n">backref</span><span class="o">=</span><span class="s">'user_keywords'</span><span class="p">),</span>
<span class="s">'keyword'</span><span class="p">:</span> <span class="n">relationship</span><span class="p">(</span><span class="n">Keyword</span><span class="p">),</span>
<span class="p">})</span>
<span class="n">user</span> <span class="o">=</span> <span class="n">User</span><span class="p">(</span><span class="s">'log'</span><span class="p">)</span>
<span class="n">kw1</span> <span class="o">=</span> <span class="n">Keyword</span><span class="p">(</span><span class="s">'new_from_blammo'</span><span class="p">)</span>
<span class="c"># Creating a UserKeyword association object will add a Keyword.</span>
<span class="c"># the "user" reference assignment in the UserKeyword() constructor</span>
<span class="c"># populates "user_keywords" via backref.</span>
<span class="n">UserKeyword</span><span class="p">(</span><span class="n">user</span><span class="p">,</span> <span class="n">kw1</span><span class="p">)</span>
<span class="c"># Accessing Keywords requires traversing UserKeywords</span>
<span class="k">print</span> <span class="n">user</span><span class="o">.</span><span class="n">user_keywords</span><span class="p">[</span><span class="mi">0</span><span class="p">]</span>
<span class="c"># <__main__.UserKeyword object at 0xb79bbbec></span>
<span class="k">print</span> <span class="n">user</span><span class="o">.</span><span class="n">user_keywords</span><span class="p">[</span><span class="mi">0</span><span class="p">]</span><span class="o">.</span><span class="n">keyword</span>
<span class="c"># Keyword('new_from_blammo')</span>
<span class="c"># Lots of work.</span>
<span class="c"># It's much easier to go through the association proxy!</span>
<span class="k">for</span> <span class="n">kw</span> <span class="ow">in</span> <span class="p">(</span><span class="n">Keyword</span><span class="p">(</span><span class="s">'its_big'</span><span class="p">),</span> <span class="n">Keyword</span><span class="p">(</span><span class="s">'its_heavy'</span><span class="p">),</span> <span class="n">Keyword</span><span class="p">(</span><span class="s">'its_wood'</span><span class="p">)):</span>
<span class="n">user</span><span class="o">.</span><span class="n">keywords</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="n">kw</span><span class="p">)</span>
<span class="k">print</span> <span class="n">user</span><span class="o">.</span><span class="n">keywords</span>
<span class="c"># [Keyword('new_from_blammo'), Keyword('its_big'), Keyword('its_heavy'), Keyword('its_wood')]</span></pre></div>
</div>
</div>
<div class="section" id="building-complex-views">
<h2>Building Complex Views<a class="headerlink" href="#building-complex-views" title="Permalink to this headline">¶</a></h2>
<div class="highlight-python"><div class="highlight"><pre><span class="n">stocks_table</span> <span class="o">=</span> <span class="n">Table</span><span class="p">(</span><span class="s">"stocks"</span><span class="p">,</span> <span class="n">meta</span><span class="p">,</span>
<span class="n">Column</span><span class="p">(</span><span class="s">'symbol'</span><span class="p">,</span> <span class="n">String</span><span class="p">(</span><span class="mi">10</span><span class="p">),</span> <span class="n">primary_key</span><span class="o">=</span><span class="bp">True</span><span class="p">),</span>
<span class="n">Column</span><span class="p">(</span><span class="s">'last_price'</span><span class="p">,</span> <span class="n">Numeric</span><span class="p">)</span>
<span class="p">)</span>
<span class="n">brokers_table</span> <span class="o">=</span> <span class="n">Table</span><span class="p">(</span><span class="s">"brokers"</span><span class="p">,</span> <span class="n">meta</span><span class="p">,</span>
<span class="n">Column</span><span class="p">(</span><span class="s">'id'</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="bp">True</span><span class="p">),</span>
<span class="n">Column</span><span class="p">(</span><span class="s">'name'</span><span class="p">,</span> <span class="n">String</span><span class="p">(</span><span class="mi">100</span><span class="p">),</span> <span class="n">nullable</span><span class="o">=</span><span class="bp">False</span><span class="p">)</span>
<span class="p">)</span>
<span class="n">holdings_table</span> <span class="o">=</span> <span class="n">Table</span><span class="p">(</span><span class="s">"holdings"</span><span class="p">,</span> <span class="n">meta</span><span class="p">,</span>
<span class="n">Column</span><span class="p">(</span><span class="s">'broker_id'</span><span class="p">,</span> <span class="n">Integer</span><span class="p">,</span> <span class="n">ForeignKey</span><span class="p">(</span><span class="s">'brokers.id'</span><span class="p">),</span> <span class="n">primary_key</span><span class="o">=</span><span class="bp">True</span><span class="p">),</span>
<span class="n">Column</span><span class="p">(</span><span class="s">'symbol'</span><span class="p">,</span> <span class="n">String</span><span class="p">(</span><span class="mi">10</span><span class="p">),</span> <span class="n">ForeignKey</span><span class="p">(</span><span class="s">'stocks.symbol'</span><span class="p">),</span> <span class="n">primary_key</span><span class="o">=</span><span class="bp">True</span><span class="p">),</span>
<span class="n">Column</span><span class="p">(</span><span class="s">'shares'</span><span class="p">,</span> <span class="n">Integer</span><span class="p">)</span>
<span class="p">)</span></pre></div>
</div>
<p>Above are three tables, modeling stocks, their brokers and the number of
shares of a stock held by each broker. This situation is quite different
from the association example above. <tt class="docutils literal"><span class="pre">shares</span></tt> is a <em>property of the
relationship</em>, an important one that we need to use all the time.</p>
<p>For this example, it would be very convenient if <tt class="docutils literal"><span class="pre">Broker</span></tt> objects had a
dictionary collection that mapped <tt class="docutils literal"><span class="pre">Stock</span></tt> instances to the shares held for
each. That’s easy:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">from</span> <span class="nn">sqlalchemy.ext.associationproxy</span> <span class="kn">import</span> <span class="n">association_proxy</span>
<span class="kn">from</span> <span class="nn">sqlalchemy.orm.collections</span> <span class="kn">import</span> <span class="n">attribute_mapped_collection</span>
<span class="k">def</span> <span class="nf">_create_holding</span><span class="p">(</span><span class="n">stock</span><span class="p">,</span> <span class="n">shares</span><span class="p">):</span>
<span class="sd">"""A creator function, constructs Holdings from Stock and share quantity."""</span>
<span class="k">return</span> <span class="n">Holding</span><span class="p">(</span><span class="n">stock</span><span class="o">=</span><span class="n">stock</span><span class="p">,</span> <span class="n">shares</span><span class="o">=</span><span class="n">shares</span><span class="p">)</span>
<span class="k">class</span> <span class="nc">Broker</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span>
<span class="k">def</span> <span class="nf">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">name</span><span class="p">):</span>
<span class="bp">self</span><span class="o">.</span><span class="n">name</span> <span class="o">=</span> <span class="n">name</span>
<span class="n">holdings</span> <span class="o">=</span> <span class="n">association_proxy</span><span class="p">(</span><span class="s">'by_stock'</span><span class="p">,</span> <span class="s">'shares'</span><span class="p">,</span> <span class="n">creator</span><span class="o">=</span><span class="n">_create_holding</span><span class="p">)</span>
<span class="k">class</span> <span class="nc">Stock</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span>
<span class="k">def</span> <span class="nf">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">symbol</span><span class="p">):</span>
<span class="bp">self</span><span class="o">.</span><span class="n">symbol</span> <span class="o">=</span> <span class="n">symbol</span>
<span class="bp">self</span><span class="o">.</span><span class="n">last_price</span> <span class="o">=</span> <span class="mi">0</span>
<span class="k">class</span> <span class="nc">Holding</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span>
<span class="k">def</span> <span class="nf">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">broker</span><span class="o">=</span><span class="bp">None</span><span class="p">,</span> <span class="n">stock</span><span class="o">=</span><span class="bp">None</span><span class="p">,</span> <span class="n">shares</span><span class="o">=</span><span class="mi">0</span><span class="p">):</span>
<span class="bp">self</span><span class="o">.</span><span class="n">broker</span> <span class="o">=</span> <span class="n">broker</span>
<span class="bp">self</span><span class="o">.</span><span class="n">stock</span> <span class="o">=</span> <span class="n">stock</span>
<span class="bp">self</span><span class="o">.</span><span class="n">shares</span> <span class="o">=</span> <span class="n">shares</span>
<span class="n">mapper</span><span class="p">(</span><span class="n">Stock</span><span class="p">,</span> <span class="n">stocks_table</span><span class="p">)</span>
<span class="n">mapper</span><span class="p">(</span><span class="n">Broker</span><span class="p">,</span> <span class="n">brokers_table</span><span class="p">,</span> <span class="n">properties</span><span class="o">=</span><span class="p">{</span>
<span class="s">'by_stock'</span><span class="p">:</span> <span class="n">relationship</span><span class="p">(</span><span class="n">Holding</span><span class="p">,</span>
<span class="n">collection_class</span><span class="o">=</span><span class="n">attribute_mapped_collection</span><span class="p">(</span><span class="s">'stock'</span><span class="p">))</span>
<span class="p">})</span>
<span class="n">mapper</span><span class="p">(</span><span class="n">Holding</span><span class="p">,</span> <span class="n">holdings_table</span><span class="p">,</span> <span class="n">properties</span><span class="o">=</span><span class="p">{</span>
<span class="s">'stock'</span><span class="p">:</span> <span class="n">relationship</span><span class="p">(</span><span class="n">Stock</span><span class="p">),</span>
<span class="s">'broker'</span><span class="p">:</span> <span class="n">relationship</span><span class="p">(</span><span class="n">Broker</span><span class="p">)</span>
<span class="p">})</span></pre></div>
</div>
<p>Above, we’ve set up the <tt class="docutils literal"><span class="pre">by_stock</span></tt> relationship collection to act as a
dictionary, using the <tt class="docutils literal"><span class="pre">.stock</span></tt> property of each Holding as a key.</p>
<p>Populating and accessing that dictionary manually is slightly inconvenient
because of the complexity of the Holdings association object:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="n">stock</span> <span class="o">=</span> <span class="n">Stock</span><span class="p">(</span><span class="s">'ZZK'</span><span class="p">)</span>
<span class="n">broker</span> <span class="o">=</span> <span class="n">Broker</span><span class="p">(</span><span class="s">'paj'</span><span class="p">)</span>
<span class="n">broker</span><span class="o">.</span><span class="n">by_stock</span><span class="p">[</span><span class="n">stock</span><span class="p">]</span> <span class="o">=</span> <span class="n">Holding</span><span class="p">(</span><span class="n">broker</span><span class="p">,</span> <span class="n">stock</span><span class="p">,</span> <span class="mi">10</span><span class="p">)</span>
<span class="k">print</span> <span class="n">broker</span><span class="o">.</span><span class="n">by_stock</span><span class="p">[</span><span class="n">stock</span><span class="p">]</span><span class="o">.</span><span class="n">shares</span>
<span class="c"># 10</span></pre></div>
</div>
<p>The <tt class="docutils literal"><span class="pre">holdings</span></tt> proxy we’ve added to the <tt class="docutils literal"><span class="pre">Broker</span></tt> class hides the details
of the <tt class="docutils literal"><span class="pre">Holding</span></tt> while also giving access to <tt class="docutils literal"><span class="pre">.shares</span></tt>:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="k">for</span> <span class="n">stock</span> <span class="ow">in</span> <span class="p">(</span><span class="n">Stock</span><span class="p">(</span><span class="s">'JEK'</span><span class="p">),</span> <span class="n">Stock</span><span class="p">(</span><span class="s">'STPZ'</span><span class="p">)):</span>
<span class="n">broker</span><span class="o">.</span><span class="n">holdings</span><span class="p">[</span><span class="n">stock</span><span class="p">]</span> <span class="o">=</span> <span class="mi">123</span>
<span class="k">for</span> <span class="n">stock</span><span class="p">,</span> <span class="n">shares</span> <span class="ow">in</span> <span class="n">broker</span><span class="o">.</span><span class="n">holdings</span><span class="o">.</span><span class="n">items</span><span class="p">():</span>
<span class="k">print</span> <span class="n">stock</span><span class="p">,</span> <span class="n">shares</span>
<span class="n">session</span><span class="o">.</span><span class="n">add</span><span class="p">(</span><span class="n">broker</span><span class="p">)</span>
<span class="n">session</span><span class="o">.</span><span class="n">commit</span><span class="p">()</span>
<span class="c"># lets take a peek at that holdings_table after committing changes to the db</span>
<span class="k">print</span> <span class="nb">list</span><span class="p">(</span><span class="n">holdings_table</span><span class="o">.</span><span class="n">select</span><span class="p">()</span><span class="o">.</span><span class="n">execute</span><span class="p">())</span>
<span class="c"># [(1, 'ZZK', 10), (1, 'JEK', 123), (1, 'STEPZ', 123)]</span></pre></div>
</div>
<p>Further examples can be found in the <tt class="docutils literal"><span class="pre">examples/</span></tt> directory in the
SQLAlchemy distribution.</p>
</div>
<div class="section" id="api">
<h2>API<a class="headerlink" href="#api" title="Permalink to this headline">¶</a></h2>
<dl class="function">
<dt id="sqlalchemy.ext.associationproxy.association_proxy">
<tt class="descclassname">sqlalchemy.ext.associationproxy.</tt><tt class="descname">association_proxy</tt><big>(</big><em>target_collection</em>, <em>attr</em>, <em>**kw</em><big>)</big><a class="headerlink" href="#sqlalchemy.ext.associationproxy.association_proxy" title="Permalink to this definition">¶</a></dt>
<dd><p>Return a Python property implementing a view of <em>attr</em> over a collection.</p>
<p>Implements a read/write view over an instance’s <em>target_collection</em>,
extracting <em>attr</em> from each member of the collection. The property acts
somewhat like this list comprehension:</p>
<div class="highlight-python"><pre>[getattr(member, *attr*)
for member in getattr(instance, *target_collection*)]</pre>
</div>
<p>Unlike the list comprehension, the collection returned by the property is
always in sync with <em>target_collection</em>, and mutations made to either
collection will be reflected in both.</p>
<p>Implements a Python property representing a relationship as a collection of
simpler values. The proxied property will mimic the collection type of
the target (list, dict or set), or, in the case of a one to one relationship,
a simple scalar value.</p>
<table class="docutils field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Parameters:</th><td class="field-body"><ul class="first last simple">
<li><strong>target_collection</strong> – Name of the relationship attribute we’ll proxy to,
usually created with <a class="reference internal" href="../relationships.html#sqlalchemy.orm.relationship" title="sqlalchemy.orm.relationship"><tt class="xref py py-func docutils literal"><span class="pre">relationship()</span></tt></a>.</li>
<li><strong>attr</strong> – <p>Attribute on the associated instances we’ll proxy for.</p>
<p>For example, given a target collection of [obj1, obj2], a list created
by this proxy property would look like [getattr(obj1, <em>attr</em>),
getattr(obj2, <em>attr</em>)]</p>
<p>If the relationship is one-to-one or otherwise uselist=False, then simply:
getattr(obj, <em>attr</em>)</p>
</li>
<li><strong>creator</strong> – <p>optional.</p>
<p>When new items are added to this proxied collection, new instances of
the class collected by the target collection will be created. For list
and set collections, the target class constructor will be called with
the ‘value’ for the new instance. For dict types, two arguments are
passed: key and value.</p>
<p>If you want to construct instances differently, supply a <em>creator</em>
function that takes arguments as above and returns instances.</p>
<p>For scalar relationships, creator() will be called if the target is None.
If the target is present, set operations are proxied to setattr() on the
associated object.</p>
<p>If you have an associated object with multiple attributes, you may set
up multiple association proxies mapping to different attributes. See
the unit tests for examples, and for examples of how creator() functions
can be used to construct the scalar relationship on-demand in this
situation.</p>
</li>
<li><strong>**kw</strong> – Passes along any other keyword arguments to
<a class="reference internal" href="#sqlalchemy.ext.associationproxy.AssociationProxy" title="sqlalchemy.ext.associationproxy.AssociationProxy"><tt class="xref py py-class docutils literal"><span class="pre">AssociationProxy</span></tt></a>.</li>
</ul>
</td>
</tr>
</tbody>
</table>
</dd></dl>
<dl class="class">
<dt id="sqlalchemy.ext.associationproxy.AssociationProxy">
<em class="property">class </em><tt class="descclassname">sqlalchemy.ext.associationproxy.</tt><tt class="descname">AssociationProxy</tt><big>(</big><em>target_collection</em>, <em>attr</em>, <em>creator=None</em>, <em>getset_factory=None</em>, <em>proxy_factory=None</em>, <em>proxy_bulk_set=None</em><big>)</big><a class="headerlink" href="#sqlalchemy.ext.associationproxy.AssociationProxy" title="Permalink to this definition">¶</a></dt>
<dd><p>A descriptor that presents a read/write view of an object attribute.</p>
<dl class="method">
<dt id="sqlalchemy.ext.associationproxy.AssociationProxy.__init__">
<tt class="descname">__init__</tt><big>(</big><em>target_collection</em>, <em>attr</em>, <em>creator=None</em>, <em>getset_factory=None</em>, <em>proxy_factory=None</em>, <em>proxy_bulk_set=None</em><big>)</big><a class="headerlink" href="#sqlalchemy.ext.associationproxy.AssociationProxy.__init__" title="Permalink to this definition">¶</a></dt>
<dd><p>Arguments are:</p>
<dl class="docutils">
<dt>target_collection</dt>
<dd>Name of the collection we’ll proxy to, usually created with
‘relationship()’ in a mapper setup.</dd>
<dt>attr</dt>
<dd>Attribute on the collected instances we’ll proxy for. For example,
given a target collection of [obj1, obj2], a list created by this
proxy property would look like [getattr(obj1, attr), getattr(obj2,
attr)]</dd>
<dt>creator</dt>
<dd><p class="first">Optional. When new items are added to this proxied collection, new
instances of the class collected by the target collection will be
created. For list and set collections, the target class constructor
will be called with the ‘value’ for the new instance. For dict
types, two arguments are passed: key and value.</p>
<p class="last">If you want to construct instances differently, supply a ‘creator’
function that takes arguments as above and returns instances.</p>
</dd>
<dt>getset_factory</dt>
<dd><p class="first">Optional. Proxied attribute access is automatically handled by
routines that get and set values based on the <cite>attr</cite> argument for
this proxy.</p>
<p class="last">If you would like to customize this behavior, you may supply a
<cite>getset_factory</cite> callable that produces a tuple of <cite>getter</cite> and
<cite>setter</cite> functions. The factory is called with two arguments, the
abstract type of the underlying collection and this proxy instance.</p>
</dd>
<dt>proxy_factory</dt>
<dd>Optional. The type of collection to emulate is determined by
sniffing the target collection. If your collection type can’t be
determined by duck typing or you’d like to use a different
collection implementation, you may supply a factory function to
produce those collections. Only applicable to non-scalar relationships.</dd>
<dt>proxy_bulk_set</dt>
<dd>Optional, use with proxy_factory. See the _set() method for
details.</dd>
</dl>
</dd></dl>
<dl class="method">
<dt id="sqlalchemy.ext.associationproxy.AssociationProxy.any">
<tt class="descname">any</tt><big>(</big><em>criterion=None</em>, <em>**kwargs</em><big>)</big><a class="headerlink" href="#sqlalchemy.ext.associationproxy.AssociationProxy.any" title="Permalink to this definition">¶</a></dt>
<dd></dd></dl>
<dl class="method">
<dt id="sqlalchemy.ext.associationproxy.AssociationProxy.contains">
<tt class="descname">contains</tt><big>(</big><em>obj</em><big>)</big><a class="headerlink" href="#sqlalchemy.ext.associationproxy.AssociationProxy.contains" title="Permalink to this definition">¶</a></dt>
<dd></dd></dl>
<dl class="method">
<dt id="sqlalchemy.ext.associationproxy.AssociationProxy.has">
<tt class="descname">has</tt><big>(</big><em>criterion=None</em>, <em>**kwargs</em><big>)</big><a class="headerlink" href="#sqlalchemy.ext.associationproxy.AssociationProxy.has" title="Permalink to this definition">¶</a></dt>
<dd></dd></dl>
<dl class="attribute">
<dt id="sqlalchemy.ext.associationproxy.AssociationProxy.target_class">
<tt class="descname">target_class</tt><a class="headerlink" href="#sqlalchemy.ext.associationproxy.AssociationProxy.target_class" title="Permalink to this definition">¶</a></dt>
<dd><p>The class the proxy is attached to.</p>
</dd></dl>
</dd></dl>
</div>
</div>
</div>
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