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examples.custom_attributes.active_column_defaults
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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.custom_attributes.active_column_defaults</h1><div class="highlight"><pre>
<span></span><span class="sd">"""Illustrates use of the :meth:`.AttributeEvents.init_scalar`</span>
<span class="sd">event, in conjunction with Core column defaults to provide</span>
<span class="sd">ORM objects that automatically produce the default value</span>
<span class="sd">when an un-set attribute is accessed.</span>
<span class="sd">"""</span>
<span class="kn">from</span> <span class="nn">sqlalchemy</span> <span class="k">import</span> <span class="n">event</span>
<span class="k">def</span> <span class="nf">configure_listener</span><span class="p">(</span><span class="n">mapper</span><span class="p">,</span> <span class="n">class_</span><span class="p">):</span>
<span class="sd">"""Establish attribute setters for every default-holding column on the</span>
<span class="sd"> given mapper."""</span>
<span class="c1"># iterate through ColumnProperty objects</span>
<span class="k">for</span> <span class="n">col_attr</span> <span class="ow">in</span> <span class="n">mapper</span><span class="o">.</span><span class="n">column_attrs</span><span class="p">:</span>
<span class="c1"># look at the Column mapped by the ColumnProperty</span>
<span class="c1"># (we look at the first column in the less common case</span>
<span class="c1"># of a property mapped to multiple columns at once)</span>
<span class="n">column</span> <span class="o">=</span> <span class="n">col_attr</span><span class="o">.</span><span class="n">columns</span><span class="p">[</span><span class="mi">0</span><span class="p">]</span>
<span class="c1"># if the Column has a "default", set up a listener</span>
<span class="k">if</span> <span class="n">column</span><span class="o">.</span><span class="n">default</span> <span class="ow">is</span> <span class="ow">not</span> <span class="kc">None</span><span class="p">:</span>
<span class="n">default_listener</span><span class="p">(</span><span class="n">col_attr</span><span class="p">,</span> <span class="n">column</span><span class="o">.</span><span class="n">default</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">default_listener</span><span class="p">(</span><span class="n">col_attr</span><span class="p">,</span> <span class="n">default</span><span class="p">):</span>
<span class="sd">"""Establish a default-setting listener.</span>
<span class="sd"> Given a class_, attrname, and a :class:`.DefaultGenerator` instance.</span>
<span class="sd"> The default generator should be a :class:`.ColumnDefault` object with a</span>
<span class="sd"> plain Python value or callable default; otherwise, the appropriate behavior</span>
<span class="sd"> for SQL functions and defaults should be determined here by the</span>
<span class="sd"> user integrating this feature.</span>
<span class="sd"> """</span>
<span class="nd">@event</span><span class="o">.</span><span class="n">listens_for</span><span class="p">(</span><span class="n">col_attr</span><span class="p">,</span> <span class="s2">"init_scalar"</span><span class="p">,</span> <span class="n">retval</span><span class="o">=</span><span class="kc">True</span><span class="p">,</span> <span class="n">propagate</span><span class="o">=</span><span class="kc">True</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">init_scalar</span><span class="p">(</span><span class="n">target</span><span class="p">,</span> <span class="n">value</span><span class="p">,</span> <span class="n">dict_</span><span class="p">):</span>
<span class="k">if</span> <span class="n">default</span><span class="o">.</span><span class="n">is_callable</span><span class="p">:</span>
<span class="c1"># the callable of ColumnDefault always accepts a context</span>
<span class="c1"># argument; we can pass it as None here.</span>
<span class="n">value</span> <span class="o">=</span> <span class="n">default</span><span class="o">.</span><span class="n">arg</span><span class="p">(</span><span class="kc">None</span><span class="p">)</span>
<span class="k">elif</span> <span class="n">default</span><span class="o">.</span><span class="n">is_scalar</span><span class="p">:</span>
<span class="n">value</span> <span class="o">=</span> <span class="n">default</span><span class="o">.</span><span class="n">arg</span>
<span class="k">else</span><span class="p">:</span>
<span class="c1"># default is a Sequence, a SQL expression, server</span>
<span class="c1"># side default generator, or other non-Python-evaluable</span>
<span class="c1"># object. The feature here can't easily support this. This</span>
<span class="c1"># can be made to return None, rather than raising,</span>
<span class="c1"># or can procure a connection from an Engine</span>
<span class="c1"># or Session and actually run the SQL, if desired.</span>
<span class="k">raise</span> <span class="ne">NotImplementedError</span><span class="p">(</span>
<span class="s2">"Can't invoke pre-default for a SQL-level column default"</span>
<span class="p">)</span>
<span class="c1"># set the value in the given dict_; this won't emit any further</span>
<span class="c1"># attribute set events or create attribute "history", but the value</span>
<span class="c1"># will be used in the INSERT statement</span>
<span class="n">dict_</span><span class="p">[</span><span class="n">col_attr</span><span class="o">.</span><span class="n">key</span><span class="p">]</span> <span class="o">=</span> <span class="n">value</span>
<span class="c1"># return the value as well</span>
<span class="k">return</span> <span class="n">value</span>
<span class="k">if</span> <span class="vm">__name__</span> <span class="o">==</span> <span class="s2">"__main__"</span><span class="p">:</span>
<span class="kn">from</span> <span class="nn">sqlalchemy</span> <span class="k">import</span> <span class="n">Column</span><span class="p">,</span> <span class="n">Integer</span><span class="p">,</span> <span class="n">DateTime</span><span class="p">,</span> <span class="n">create_engine</span>
<span class="kn">from</span> <span class="nn">sqlalchemy.orm</span> <span class="k">import</span> <span class="n">Session</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">import</span> <span class="nn">datetime</span>
<span class="n">Base</span> <span class="o">=</span> <span class="n">declarative_base</span><span class="p">()</span>
<span class="n">event</span><span class="o">.</span><span class="n">listen</span><span class="p">(</span><span class="n">Base</span><span class="p">,</span> <span class="s2">"mapper_configured"</span><span class="p">,</span> <span class="n">configure_listener</span><span class="p">,</span> <span class="n">propagate</span><span class="o">=</span><span class="kc">True</span><span class="p">)</span>
<span class="k">class</span> <span class="nc">Widget</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="s2">"widget"</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">radius</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">default</span><span class="o">=</span><span class="mi">30</span><span class="p">)</span>
<span class="n">timestamp</span> <span class="o">=</span> <span class="n">Column</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">datetime</span><span class="o">.</span><span class="n">now</span><span class="p">)</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">w1</span> <span class="o">=</span> <span class="n">Widget</span><span class="p">()</span>
<span class="c1"># not persisted at all, default values are present the moment</span>
<span class="c1"># we access them</span>
<span class="k">assert</span> <span class="n">w1</span><span class="o">.</span><span class="n">radius</span> <span class="o">==</span> <span class="mi">30</span>
<span class="c1"># this line will invoke the datetime.now() function, and establish</span>
<span class="c1"># its return value upon the w1 instance, such that the</span>
<span class="c1"># Column-level default for the "timestamp" column will no longer fire</span>
<span class="c1"># off.</span>
<span class="n">current_time</span> <span class="o">=</span> <span class="n">w1</span><span class="o">.</span><span class="n">timestamp</span>
<span class="k">assert</span> <span class="n">current_time</span> <span class="o">></span> <span class="n">datetime</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="o">-</span> <span class="n">datetime</span><span class="o">.</span><span class="n">timedelta</span><span class="p">(</span>
<span class="n">seconds</span><span class="o">=</span><span class="mi">5</span>
<span class="p">)</span>
<span class="c1"># persist</span>
<span class="n">sess</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">sess</span><span class="o">.</span><span class="n">add</span><span class="p">(</span><span class="n">w1</span><span class="p">)</span>
<span class="n">sess</span><span class="o">.</span><span class="n">commit</span><span class="p">()</span>
<span class="c1"># data is persisted. The timestamp is also the one we generated above;</span>
<span class="c1"># e.g. the default wasn't re-invoked later.</span>
<span class="k">assert</span> <span class="n">sess</span><span class="o">.</span><span class="n">query</span><span class="p">(</span><span class="n">Widget</span><span class="o">.</span><span class="n">radius</span><span class="p">,</span> <span class="n">Widget</span><span class="o">.</span><span class="n">timestamp</span><span class="p">)</span><span class="o">.</span><span class="n">first</span><span class="p">()</span> <span class="o">==</span> <span class="p">(</span>
<span class="mi">30</span><span class="p">,</span>
<span class="n">current_time</span><span class="p">,</span>
<span class="p">)</span></pre></div>
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