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examples.join_conditions.cast
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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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<h1>Source code for examples.join_conditions.cast</h1><div class="highlight"><pre>
<span></span><span class="sd">"""Illustrate a :func:`.relationship` that joins two columns where those</span>
<span class="sd">columns are not of the same type, and a CAST must be used on the SQL</span>
<span class="sd">side in order to match them.</span>
<span class="sd">When complete, we'd like to see a load of the relationship to look like::</span>
<span class="sd"> -- load the primary row, a_id is a string</span>
<span class="sd"> SELECT a.id AS a_id_1, a.a_id AS a_a_id</span>
<span class="sd"> FROM a</span>
<span class="sd"> WHERE a.a_id = '2'</span>
<span class="sd"> -- then load the collection using CAST, b.a_id is an integer</span>
<span class="sd"> SELECT b.id AS b_id, b.a_id AS b_a_id</span>
<span class="sd"> FROM b</span>
<span class="sd"> WHERE CAST('2' AS INTEGER) = b.a_id</span>
<span class="sd">The relationship is essentially configured as follows::</span>
<span class="sd"> class B(Base):</span>
<span class="sd"> # ...</span>
<span class="sd"> a = relationship(A,</span>
<span class="sd"> primaryjoin=cast(A.a_id, Integer) == foreign(B.a_id),</span>
<span class="sd"> backref="bs")</span>
<span class="sd">Where above, we are making use of the :func:`.cast` function in order</span>
<span class="sd">to produce CAST, as well as the :func:`.foreign` :term:`annotation` function</span>
<span class="sd">in order to note to the ORM that ``B.a_id`` should be treated like the</span>
<span class="sd">"foreign key" column.</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">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</span> <span class="k">import</span> <span class="n">TypeDecorator</span>
<span class="kn">from</span> <span class="nn">sqlalchemy.ext.declarative</span> <span class="k">import</span> <span class="n">declarative_base</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="n">Base</span> <span class="o">=</span> <span class="n">declarative_base</span><span class="p">()</span>
<span class="k">class</span> <span class="nc">StringAsInt</span><span class="p">(</span><span class="n">TypeDecorator</span><span class="p">):</span>
<span class="sd">"""Coerce string->integer type.</span>
<span class="sd"> This is needed only if the relationship() from</span>
<span class="sd"> int to string is writable, as SQLAlchemy will copy</span>
<span class="sd"> the string parent values into the integer attribute</span>
<span class="sd"> on the child during a flush.</span>
<span class="sd"> """</span>
<span class="n">impl</span> <span class="o">=</span> <span class="n">Integer</span>
<span class="k">def</span> <span class="nf">process_bind_param</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">value</span><span class="p">,</span> <span class="n">dialect</span><span class="p">):</span>
<span class="k">if</span> <span class="n">value</span> <span class="ow">is</span> <span class="ow">not</span> <span class="kc">None</span><span class="p">:</span>
<span class="n">value</span> <span class="o">=</span> <span class="nb">int</span><span class="p">(</span><span class="n">value</span><span class="p">)</span>
<span class="k">return</span> <span class="n">value</span>
<span class="k">class</span> <span class="nc">A</span><span class="p">(</span><span class="n">Base</span><span class="p">):</span>
<span class="sd">"""Parent. The referenced column is a string type."""</span>
<span class="n">__tablename__</span> <span class="o">=</span> <span class="s2">"a"</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="n">a_id</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">B</span><span class="p">(</span><span class="n">Base</span><span class="p">):</span>
<span class="sd">"""Child. The column we reference 'A' with is an integer."""</span>
<span class="n">__tablename__</span> <span class="o">=</span> <span class="s2">"b"</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="n">a_id</span> <span class="o">=</span> <span class="n">Column</span><span class="p">(</span><span class="n">StringAsInt</span><span class="p">)</span>
<span class="n">a</span> <span class="o">=</span> <span class="n">relationship</span><span class="p">(</span>
<span class="s2">"A"</span><span class="p">,</span>
<span class="c1"># specify primaryjoin. The string form is optional</span>
<span class="c1"># here, but note that Declarative makes available all</span>
<span class="c1"># of the built-in functions we might need, including</span>
<span class="c1"># cast() and foreign().</span>
<span class="n">primaryjoin</span><span class="o">=</span><span class="s2">"cast(A.a_id, Integer) == foreign(B.a_id)"</span><span class="p">,</span>
<span class="n">backref</span><span class="o">=</span><span class="s2">"bs"</span><span class="p">,</span>
<span class="p">)</span>
<span class="c1"># we demonstrate with SQLite, but the important part</span>
<span class="c1"># is the CAST rendered in the SQL output.</span>
<span class="n">e</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">e</span><span class="p">)</span>
<span class="n">s</span> <span class="o">=</span> <span class="n">Session</span><span class="p">(</span><span class="n">e</span><span class="p">)</span>
<span class="n">s</span><span class="o">.</span><span class="n">add_all</span><span class="p">([</span><span class="n">A</span><span class="p">(</span><span class="n">a_id</span><span class="o">=</span><span class="s2">"1"</span><span class="p">),</span> <span class="n">A</span><span class="p">(</span><span class="n">a_id</span><span class="o">=</span><span class="s2">"2"</span><span class="p">,</span> <span class="n">bs</span><span class="o">=</span><span class="p">[</span><span class="n">B</span><span class="p">(),</span> <span class="n">B</span><span class="p">()]),</span> <span class="n">A</span><span class="p">(</span><span class="n">a_id</span><span class="o">=</span><span class="s2">"3"</span><span class="p">,</span> <span class="n">bs</span><span class="o">=</span><span class="p">[</span><span class="n">B</span><span class="p">()])])</span>
<span class="n">s</span><span class="o">.</span><span class="n">commit</span><span class="p">()</span>
<span class="n">b1</span> <span class="o">=</span> <span class="n">s</span><span class="o">.</span><span class="n">query</span><span class="p">(</span><span class="n">B</span><span class="p">)</span><span class="o">.</span><span class="n">filter_by</span><span class="p">(</span><span class="n">a_id</span><span class="o">=</span><span class="s2">"2"</span><span class="p">)</span><span class="o">.</span><span class="n">first</span><span class="p">()</span>
<span class="nb">print</span><span class="p">(</span><span class="n">b1</span><span class="o">.</span><span class="n">a</span><span class="p">)</span>
<span class="n">a1</span> <span class="o">=</span> <span class="n">s</span><span class="o">.</span><span class="n">query</span><span class="p">(</span><span class="n">A</span><span class="p">)</span><span class="o">.</span><span class="n">filter_by</span><span class="p">(</span><span class="n">a_id</span><span class="o">=</span><span class="s2">"2"</span><span class="p">)</span><span class="o">.</span><span class="n">first</span><span class="p">()</span>
<span class="nb">print</span><span class="p">(</span><span class="n">a1</span><span class="o">.</span><span class="n">bs</span><span class="p">)</span></pre></div>
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