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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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<h1>Source code for examples.generic_associations.discriminator_on_association</h1><div class="highlight"><pre>
<span></span><span class="sd">"""Illustrates a mixin which provides a generic association</span>
<span class="sd">using a single target table and a single association table,</span>
<span class="sd">referred to by all parent tables. The association table</span>
<span class="sd">contains a "discriminator" column which determines what type of</span>
<span class="sd">parent object associates to each particular row in the association</span>
<span class="sd">table.</span>
<span class="sd">SQLAlchemy's single-table-inheritance feature is used</span>
<span class="sd">to target different association types.</span>
<span class="sd">This configuration attempts to simulate a so-called "generic foreign key"</span>
<span class="sd">as closely as possible without actually foregoing the use of real</span>
<span class="sd">foreign keys. Unlike table-per-related and table-per-association,</span>
<span class="sd">it uses a fixed number of tables to serve any number of potential parent</span>
<span class="sd">objects, but is also slightly more complex.</span>
<span class="sd">"""</span>
<span class="kn">from</span> <span class="nn">sqlalchemy</span> <span class="k">import</span> <span class="n">Column</span>
<span class="kn">from</span> <span class="nn">sqlalchemy</span> <span class="k">import</span> <span class="n">create_engine</span>
<span class="kn">from</span> <span class="nn">sqlalchemy</span> <span class="k">import</span> <span class="n">ForeignKey</span>
<span class="kn">from</span> <span class="nn">sqlalchemy</span> <span class="k">import</span> <span class="n">Integer</span>
<span class="kn">from</span> <span class="nn">sqlalchemy</span> <span class="k">import</span> <span class="n">String</span>
<span class="kn">from</span> <span class="nn">sqlalchemy.ext.associationproxy</span> <span class="k">import</span> <span class="n">association_proxy</span>
<span class="kn">from</span> <span class="nn">sqlalchemy.ext.declarative</span> <span class="k">import</span> <span class="n">as_declarative</span>
<span class="kn">from</span> <span class="nn">sqlalchemy.ext.declarative</span> <span class="k">import</span> <span class="n">declared_attr</span>
<span class="kn">from</span> <span class="nn">sqlalchemy.orm</span> <span class="k">import</span> <span class="n">backref</span>
<span class="kn">from</span> <span class="nn">sqlalchemy.orm</span> <span class="k">import</span> <span class="n">relationship</span>
<span class="kn">from</span> <span class="nn">sqlalchemy.orm</span> <span class="k">import</span> <span class="n">Session</span>
<span class="nd">@as_declarative</span><span class="p">()</span>
<span class="k">class</span> <span class="nc">Base</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span>
<span class="sd">"""Base class which provides automated table name</span>
<span class="sd"> and surrogate primary key column.</span>
<span class="sd"> """</span>
<span class="nd">@declared_attr</span>
<span class="k">def</span> <span class="nf">__tablename__</span><span class="p">(</span><span class="bp">cls</span><span class="p">):</span>
<span class="k">return</span> <span class="bp">cls</span><span class="o">.</span><span class="vm">__name__</span><span class="o">.</span><span class="n">lower</span><span class="p">()</span>
<span class="nb">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="k">class</span> <span class="nc">AddressAssociation</span><span class="p">(</span><span class="n">Base</span><span class="p">):</span>
<span class="sd">"""Associates a collection of Address objects</span>
<span class="sd"> with a particular parent.</span>
<span class="sd"> """</span>
<span class="n">__tablename__</span> <span class="o">=</span> <span class="s2">"address_association"</span>
<span class="n">discriminator</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="sd">"""Refers to the type of parent."""</span>
<span class="n">__mapper_args__</span> <span class="o">=</span> <span class="p">{</span><span class="s2">"polymorphic_on"</span><span class="p">:</span> <span class="n">discriminator</span><span class="p">}</span>
<span class="k">class</span> <span class="nc">Address</span><span class="p">(</span><span class="n">Base</span><span class="p">):</span>
<span class="sd">"""The Address class.</span>
<span class="sd"> This represents all address records in a</span>
<span class="sd"> single table.</span>
<span class="sd"> """</span>
<span class="n">association_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">ForeignKey</span><span class="p">(</span><span class="s2">"address_association.id"</span><span class="p">))</span>
<span class="n">street</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="n">city</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="nb">zip</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="n">association</span> <span class="o">=</span> <span class="n">relationship</span><span class="p">(</span><span class="s2">"AddressAssociation"</span><span class="p">,</span> <span class="n">backref</span><span class="o">=</span><span class="s2">"addresses"</span><span class="p">)</span>
<span class="n">parent</span> <span class="o">=</span> <span class="n">association_proxy</span><span class="p">(</span><span class="s2">"association"</span><span class="p">,</span> <span class="s2">"parent"</span><span class="p">)</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="s2">"</span><span class="si">%s</span><span class="s2">(street=</span><span class="si">%r</span><span class="s2">, city=</span><span class="si">%r</span><span class="s2">, zip=</span><span class="si">%r</span><span class="s2">)"</span> <span class="o">%</span> <span class="p">(</span>
<span class="bp">self</span><span class="o">.</span><span class="vm">__class__</span><span class="o">.</span><span class="vm">__name__</span><span class="p">,</span>
<span class="bp">self</span><span class="o">.</span><span class="n">street</span><span class="p">,</span>
<span class="bp">self</span><span class="o">.</span><span class="n">city</span><span class="p">,</span>
<span class="bp">self</span><span class="o">.</span><span class="n">zip</span><span class="p">,</span>
<span class="p">)</span>
<span class="k">class</span> <span class="nc">HasAddresses</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span>
<span class="sd">"""HasAddresses mixin, creates a relationship to</span>
<span class="sd"> the address_association table for each parent.</span>
<span class="sd"> """</span>
<span class="nd">@declared_attr</span>
<span class="k">def</span> <span class="nf">address_association_id</span><span class="p">(</span><span class="bp">cls</span><span class="p">):</span>
<span class="k">return</span> <span class="n">Column</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="s2">"address_association.id"</span><span class="p">))</span>
<span class="nd">@declared_attr</span>
<span class="k">def</span> <span class="nf">address_association</span><span class="p">(</span><span class="bp">cls</span><span class="p">):</span>
<span class="n">name</span> <span class="o">=</span> <span class="bp">cls</span><span class="o">.</span><span class="vm">__name__</span>
<span class="n">discriminator</span> <span class="o">=</span> <span class="n">name</span><span class="o">.</span><span class="n">lower</span><span class="p">()</span>
<span class="n">assoc_cls</span> <span class="o">=</span> <span class="nb">type</span><span class="p">(</span>
<span class="s2">"</span><span class="si">%s</span><span class="s2">AddressAssociation"</span> <span class="o">%</span> <span class="n">name</span><span class="p">,</span>
<span class="p">(</span><span class="n">AddressAssociation</span><span class="p">,),</span>
<span class="nb">dict</span><span class="p">(</span>
<span class="n">__tablename__</span><span class="o">=</span><span class="kc">None</span><span class="p">,</span>
<span class="n">__mapper_args__</span><span class="o">=</span><span class="p">{</span><span class="s2">"polymorphic_identity"</span><span class="p">:</span> <span class="n">discriminator</span><span class="p">},</span>
<span class="p">),</span>
<span class="p">)</span>
<span class="bp">cls</span><span class="o">.</span><span class="n">addresses</span> <span class="o">=</span> <span class="n">association_proxy</span><span class="p">(</span>
<span class="s2">"address_association"</span><span class="p">,</span>
<span class="s2">"addresses"</span><span class="p">,</span>
<span class="n">creator</span><span class="o">=</span><span class="k">lambda</span> <span class="n">addresses</span><span class="p">:</span> <span class="n">assoc_cls</span><span class="p">(</span><span class="n">addresses</span><span class="o">=</span><span class="n">addresses</span><span class="p">),</span>
<span class="p">)</span>
<span class="k">return</span> <span class="n">relationship</span><span class="p">(</span>
<span class="n">assoc_cls</span><span class="p">,</span> <span class="n">backref</span><span class="o">=</span><span class="n">backref</span><span class="p">(</span><span class="s2">"parent"</span><span class="p">,</span> <span class="n">uselist</span><span class="o">=</span><span class="kc">False</span><span class="p">)</span>
<span class="p">)</span>
<span class="k">class</span> <span class="nc">Customer</span><span class="p">(</span><span class="n">HasAddresses</span><span class="p">,</span> <span class="n">Base</span><span class="p">):</span>
<span class="n">name</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="k">class</span> <span class="nc">Supplier</span><span class="p">(</span><span class="n">HasAddresses</span><span class="p">,</span> <span class="n">Base</span><span class="p">):</span>
<span class="n">company_name</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="n">engine</span> <span class="o">=</span> <span class="n">create_engine</span><span class="p">(</span><span class="s2">"sqlite://"</span><span class="p">,</span> <span class="n">echo</span><span class="o">=</span><span class="kc">True</span><span class="p">)</span>
<span class="n">Base</span><span class="o">.</span><span class="n">metadata</span><span class="o">.</span><span class="n">create_all</span><span class="p">(</span><span class="n">engine</span><span class="p">)</span>
<span class="n">session</span> <span class="o">=</span> <span class="n">Session</span><span class="p">(</span><span class="n">engine</span><span class="p">)</span>
<span class="n">session</span><span class="o">.</span><span class="n">add_all</span><span class="p">(</span>
<span class="p">[</span>
<span class="n">Customer</span><span class="p">(</span>
<span class="n">name</span><span class="o">=</span><span class="s2">"customer 1"</span><span class="p">,</span>
<span class="n">addresses</span><span class="o">=</span><span class="p">[</span>
<span class="n">Address</span><span class="p">(</span>
<span class="n">street</span><span class="o">=</span><span class="s2">"123 anywhere street"</span><span class="p">,</span> <span class="n">city</span><span class="o">=</span><span class="s2">"New York"</span><span class="p">,</span> <span class="nb">zip</span><span class="o">=</span><span class="s2">"10110"</span>
<span class="p">),</span>
<span class="n">Address</span><span class="p">(</span>
<span class="n">street</span><span class="o">=</span><span class="s2">"40 main street"</span><span class="p">,</span> <span class="n">city</span><span class="o">=</span><span class="s2">"San Francisco"</span><span class="p">,</span> <span class="nb">zip</span><span class="o">=</span><span class="s2">"95732"</span>
<span class="p">),</span>
<span class="p">],</span>
<span class="p">),</span>
<span class="n">Supplier</span><span class="p">(</span>
<span class="n">company_name</span><span class="o">=</span><span class="s2">"Ace Hammers"</span><span class="p">,</span>
<span class="n">addresses</span><span class="o">=</span><span class="p">[</span>
<span class="n">Address</span><span class="p">(</span><span class="n">street</span><span class="o">=</span><span class="s2">"2569 west elm"</span><span class="p">,</span> <span class="n">city</span><span class="o">=</span><span class="s2">"Detroit"</span><span class="p">,</span> <span class="nb">zip</span><span class="o">=</span><span class="s2">"56785"</span><span class="p">)</span>
<span class="p">],</span>
<span class="p">),</span>
<span class="p">]</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="k">for</span> <span class="n">customer</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">Customer</span><span class="p">):</span>
<span class="k">for</span> <span class="n">address</span> <span class="ow">in</span> <span class="n">customer</span><span class="o">.</span><span class="n">addresses</span><span class="p">:</span>
<span class="nb">print</span><span class="p">(</span><span class="n">address</span><span class="p">)</span>
<span class="nb">print</span><span class="p">(</span><span class="n">address</span><span class="o">.</span><span class="n">parent</span><span class="p">)</span></pre></div>
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