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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.dogpile_caching.local_session_caching</h1><div class="highlight"><pre>
<span></span><span class="sd">"""This example creates a new dogpile.cache backend that will persist data in a</span>
<span class="sd">dictionary which is local to the current session. remove() the session and</span>
<span class="sd">the cache is gone.</span>
<span class="sd">Create a new Dogpile cache backend that will store</span>
<span class="sd">cached data local to the current Session.</span>
<span class="sd">This is an advanced example which assumes familiarity</span>
<span class="sd">with the basic operation of CachingQuery.</span>
<span class="sd">"""</span>
<span class="kn">from</span> <span class="nn">dogpile.cache.api</span> <span class="k">import</span> <span class="n">CacheBackend</span>
<span class="kn">from</span> <span class="nn">dogpile.cache.api</span> <span class="k">import</span> <span class="n">NO_VALUE</span>
<span class="kn">from</span> <span class="nn">dogpile.cache.region</span> <span class="k">import</span> <span class="n">register_backend</span>
<span class="k">class</span> <span class="nc">ScopedSessionBackend</span><span class="p">(</span><span class="n">CacheBackend</span><span class="p">):</span>
<span class="sd">"""A dogpile backend which will cache objects locally on</span>
<span class="sd"> the current session.</span>
<span class="sd"> When used with the query_cache system, the effect is that the objects</span>
<span class="sd"> in the cache are the same as that within the session - the merge()</span>
<span class="sd"> is a formality that doesn't actually create a second instance.</span>
<span class="sd"> This makes it safe to use for updates of data from an identity</span>
<span class="sd"> perspective (still not ideal for deletes though).</span>
<span class="sd"> When the session is removed, the cache is gone too, so the cache</span>
<span class="sd"> is automatically disposed upon session.remove().</span>
<span class="sd"> """</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">arguments</span><span class="p">):</span>
<span class="bp">self</span><span class="o">.</span><span class="n">scoped_session</span> <span class="o">=</span> <span class="n">arguments</span><span class="p">[</span><span class="s2">"scoped_session"</span><span class="p">]</span>
<span class="k">def</span> <span class="nf">get</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">key</span><span class="p">):</span>
<span class="k">return</span> <span class="bp">self</span><span class="o">.</span><span class="n">_cache_dictionary</span><span class="o">.</span><span class="n">get</span><span class="p">(</span><span class="n">key</span><span class="p">,</span> <span class="n">NO_VALUE</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">set</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">key</span><span class="p">,</span> <span class="n">value</span><span class="p">):</span>
<span class="bp">self</span><span class="o">.</span><span class="n">_cache_dictionary</span><span class="p">[</span><span class="n">key</span><span class="p">]</span> <span class="o">=</span> <span class="n">value</span>
<span class="k">def</span> <span class="nf">delete</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">key</span><span class="p">):</span>
<span class="bp">self</span><span class="o">.</span><span class="n">_cache_dictionary</span><span class="o">.</span><span class="n">pop</span><span class="p">(</span><span class="n">key</span><span class="p">,</span> <span class="kc">None</span><span class="p">)</span>
<span class="nd">@property</span>
<span class="k">def</span> <span class="nf">_cache_dictionary</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="sd">"""Return the cache dictionary linked to the current Session."""</span>
<span class="n">sess</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">scoped_session</span><span class="p">()</span>
<span class="k">try</span><span class="p">:</span>
<span class="n">cache_dict</span> <span class="o">=</span> <span class="n">sess</span><span class="o">.</span><span class="n">_cache_dictionary</span>
<span class="k">except</span> <span class="ne">AttributeError</span><span class="p">:</span>
<span class="n">sess</span><span class="o">.</span><span class="n">_cache_dictionary</span> <span class="o">=</span> <span class="n">cache_dict</span> <span class="o">=</span> <span class="p">{}</span>
<span class="k">return</span> <span class="n">cache_dict</span>
<span class="n">register_backend</span><span class="p">(</span><span class="s2">"sqlalchemy.session"</span><span class="p">,</span> <span class="vm">__name__</span><span class="p">,</span> <span class="s2">"ScopedSessionBackend"</span><span class="p">)</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">.environment</span> <span class="k">import</span> <span class="n">Session</span><span class="p">,</span> <span class="n">regions</span>
<span class="kn">from</span> <span class="nn">.caching_query</span> <span class="k">import</span> <span class="n">FromCache</span>
<span class="kn">from</span> <span class="nn">dogpile.cache</span> <span class="k">import</span> <span class="n">make_region</span>
<span class="c1"># set up a region based on the ScopedSessionBackend,</span>
<span class="c1"># pointing to the scoped_session declared in the example</span>
<span class="c1"># environment.</span>
<span class="n">regions</span><span class="p">[</span><span class="s2">"local_session"</span><span class="p">]</span> <span class="o">=</span> <span class="n">make_region</span><span class="p">()</span><span class="o">.</span><span class="n">configure</span><span class="p">(</span>
<span class="s2">"sqlalchemy.session"</span><span class="p">,</span> <span class="n">arguments</span><span class="o">=</span><span class="p">{</span><span class="s2">"scoped_session"</span><span class="p">:</span> <span class="n">Session</span><span class="p">}</span>
<span class="p">)</span>
<span class="kn">from</span> <span class="nn">.model</span> <span class="k">import</span> <span class="n">Person</span>
<span class="c1"># query to load Person by name, with criterion</span>
<span class="c1"># of "person 10"</span>
<span class="n">q</span> <span class="o">=</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">Person</span><span class="p">)</span>
<span class="o">.</span><span class="n">options</span><span class="p">(</span><span class="n">FromCache</span><span class="p">(</span><span class="s2">"local_session"</span><span class="p">))</span>
<span class="o">.</span><span class="n">filter</span><span class="p">(</span><span class="n">Person</span><span class="o">.</span><span class="n">name</span> <span class="o">==</span> <span class="s2">"person 10"</span><span class="p">)</span>
<span class="p">)</span>
<span class="c1"># load from DB</span>
<span class="n">person10</span> <span class="o">=</span> <span class="n">q</span><span class="o">.</span><span class="n">one</span><span class="p">()</span>
<span class="c1"># next call, the query is cached.</span>
<span class="n">person10</span> <span class="o">=</span> <span class="n">q</span><span class="o">.</span><span class="n">one</span><span class="p">()</span>
<span class="c1"># clear out the Session. The "_cache_dictionary" dictionary</span>
<span class="c1"># disappears with it.</span>
<span class="n">Session</span><span class="o">.</span><span class="n">remove</span><span class="p">()</span>
<span class="c1"># query calls from DB again</span>
<span class="n">person10</span> <span class="o">=</span> <span class="n">q</span><span class="o">.</span><span class="n">one</span><span class="p">()</span>
<span class="c1"># identity is preserved - person10 is the *same* object that's</span>
<span class="c1"># ultimately inside the cache. So it is safe to manipulate</span>
<span class="c1"># the not-queried-for attributes of objects when using such a</span>
<span class="c1"># cache without the need to invalidate - however, any change</span>
<span class="c1"># that would change the results of a cached query, such as</span>
<span class="c1"># inserts, deletes, or modification to attributes that are</span>
<span class="c1"># part of query criterion, still require careful invalidation.</span>
<span class="n">cache</span><span class="p">,</span> <span class="n">key</span> <span class="o">=</span> <span class="n">q</span><span class="o">.</span><span class="n">_get_cache_plus_key</span><span class="p">()</span>
<span class="k">assert</span> <span class="n">person10</span> <span class="ow">is</span> <span class="n">cache</span><span class="o">.</span><span class="n">get</span><span class="p">(</span><span class="n">key</span><span class="p">)[</span><span class="mi">0</span><span class="p">]</span></pre></div>
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