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<h1 id="use-case-10-caching-population-read-through-populated-on-read">Use Case 10: Caching + Population (Read-Through, Populated on Read)</h1>
<p><strong>Scenario:</strong> A service needs fast reads of a <code>User</code> <strong>and</strong> its related <code>Profile</code>. We want the cache-hit path to skip the DB entirely while still returning a populated document.</p>
<hr />
<h2 id="whats-new">📦 What's New?</h2>
<table>
<thead>
<tr>
<th>Component</th>
<th>Description</th>
</tr>
</thead>
<tbody>
<tr>
<td><code>CachedBaseRepository</code></td>
<td>Provides the cache layer (<code>cache-first get_by_id</code>, invalidation on <code>update</code>/<code>delete</code>).</td>
</tr>
<tr>
<td><code>PopulationEngine</code></td>
<td>Provides the reference resolution.</td>
</tr>
<tr>
<td><strong>Composed subclass</strong></td>
<td>A small <code>CachedBaseRepository</code> subclass adds two <code>_populate</code> helpers so cache hits return populated models.</td>
</tr>
</tbody>
</table>
<blockquote>
<p>⚠️ <strong>Known limitation:</strong> there is <strong>no built-in <code>CachedPopulatingRepository</code></strong> in the library. This use case documents the composition. We cache the <strong>raw (depopulated)</strong> document — references stay <code>ObjectId</code>s until read time, so <code>Profile</code> changes are reflected on the next fetch (within TTL) and <code>update</code>/<code>delete</code> invalidation remains correct.</p>
</blockquote>
<hr />
<h2 id="example">🚀 Example</h2>
<div class="language-python highlight"><span class="filename">Python</span><pre><span></span><code><a id="__codelineno-0-1" name="__codelineno-0-1" href="#__codelineno-0-1"></a><span class="kn">from</span><span class="w"> </span><span class="nn">contextlib</span><span class="w"> </span><span class="kn">import</span> <span class="n">asynccontextmanager</span>
<a id="__codelineno-0-2" name="__codelineno-0-2" href="#__codelineno-0-2"></a><span class="kn">from</span><span class="w"> </span><span class="nn">datetime</span><span class="w"> </span><span class="kn">import</span> <span class="n">datetime</span>
<a id="__codelineno-0-3" name="__codelineno-0-3" href="#__codelineno-0-3"></a><span class="kn">from</span><span class="w"> </span><span class="nn">typing</span><span class="w"> </span><span class="kn">import</span> <span class="n">Any</span><span class="p">,</span> <span class="n">Union</span>
<a id="__codelineno-0-4" name="__codelineno-0-4" href="#__codelineno-0-4"></a>
<a id="__codelineno-0-5" name="__codelineno-0-5" href="#__codelineno-0-5"></a><span class="kn">from</span><span class="w"> </span><span class="nn">bson</span><span class="w"> </span><span class="kn">import</span> <span class="n">ObjectId</span>
<a id="__codelineno-0-6" name="__codelineno-0-6" href="#__codelineno-0-6"></a><span class="kn">from</span><span class="w"> </span><span class="nn">fastapi</span><span class="w"> </span><span class="kn">import</span> <span class="n">FastAPI</span>
<a id="__codelineno-0-7" name="__codelineno-0-7" href="#__codelineno-0-7"></a><span class="kn">from</span><span class="w"> </span><span class="nn">mongo_ops</span><span class="w"> </span><span class="kn">import</span> <span class="n">BaseDocument</span><span class="p">,</span> <span class="n">CachedBaseRepository</span><span class="p">,</span> <span class="n">ModelRegistry</span><span class="p">,</span> <span class="n">MongoConnectionManager</span>
<a id="__codelineno-0-8" name="__codelineno-0-8" href="#__codelineno-0-8"></a><span class="kn">from</span><span class="w"> </span><span class="nn">mongo_ops.cache</span><span class="w"> </span><span class="kn">import</span> <span class="n">CacheConfig</span><span class="p">,</span> <span class="n">InMemoryCacheBackend</span>
<a id="__codelineno-0-9" name="__codelineno-0-9" href="#__codelineno-0-9"></a><span class="kn">from</span><span class="w"> </span><span class="nn">mongo_ops.cache.in_memory</span><span class="w"> </span><span class="kn">import</span> <span class="n">decode_value</span><span class="p">,</span> <span class="n">encode_value</span>
<a id="__codelineno-0-10" name="__codelineno-0-10" href="#__codelineno-0-10"></a><span class="kn">from</span><span class="w"> </span><span class="nn">mongo_ops.populate</span><span class="w"> </span><span class="kn">import</span> <span class="n">PopulateRule</span><span class="p">,</span> <span class="n">PopulationEngine</span>
<a id="__codelineno-0-11" name="__codelineno-0-11" href="#__codelineno-0-11"></a>
<a id="__codelineno-0-12" name="__codelineno-0-12" href="#__codelineno-0-12"></a>
<a id="__codelineno-0-13" name="__codelineno-0-13" href="#__codelineno-0-13"></a><span class="c1"># 1. Models — `profile` holds an ObjectId in DB, a Profile in memory.</span>
<a id="__codelineno-0-14" name="__codelineno-0-14" href="#__codelineno-0-14"></a><span class="k">class</span><span class="w"> </span><span class="nc">Profile</span><span class="p">(</span><span class="n">BaseDocument</span><span class="p">):</span>
<a id="__codelineno-0-15" name="__codelineno-0-15" href="#__codelineno-0-15"></a> <span class="n">avatar_url</span><span class="p">:</span> <span class="nb">str</span> <span class="o">=</span> <span class="s2">&quot;&quot;</span>
<a id="__codelineno-0-16" name="__codelineno-0-16" href="#__codelineno-0-16"></a> <span class="n">bio</span><span class="p">:</span> <span class="nb">str</span> <span class="o">=</span> <span class="s2">&quot;&quot;</span>
<a id="__codelineno-0-17" name="__codelineno-0-17" href="#__codelineno-0-17"></a>
<a id="__codelineno-0-18" name="__codelineno-0-18" href="#__codelineno-0-18"></a>
<a id="__codelineno-0-19" name="__codelineno-0-19" href="#__codelineno-0-19"></a><span class="k">class</span><span class="w"> </span><span class="nc">User</span><span class="p">(</span><span class="n">BaseDocument</span><span class="p">):</span>
<a id="__codelineno-0-20" name="__codelineno-0-20" href="#__codelineno-0-20"></a> <span class="n">username</span><span class="p">:</span> <span class="nb">str</span> <span class="o">=</span> <span class="s2">&quot;&quot;</span>
<a id="__codelineno-0-21" name="__codelineno-0-21" href="#__codelineno-0-21"></a> <span class="n">email</span><span class="p">:</span> <span class="nb">str</span> <span class="o">=</span> <span class="s2">&quot;&quot;</span>
<a id="__codelineno-0-22" name="__codelineno-0-22" href="#__codelineno-0-22"></a> <span class="n">profile</span><span class="p">:</span> <span class="n">Profile</span> <span class="o">|</span> <span class="kc">None</span> <span class="o">=</span> <span class="kc">None</span>
<a id="__codelineno-0-23" name="__codelineno-0-23" href="#__codelineno-0-23"></a>
<a id="__codelineno-0-24" name="__codelineno-0-24" href="#__codelineno-0-24"></a>
<a id="__codelineno-0-25" name="__codelineno-0-25" href="#__codelineno-0-25"></a><span class="c1"># 2. Engine + rule.</span>
<a id="__codelineno-0-26" name="__codelineno-0-26" href="#__codelineno-0-26"></a><span class="n">engine</span> <span class="o">=</span> <span class="n">PopulationEngine</span><span class="p">({})</span>
<a id="__codelineno-0-27" name="__codelineno-0-27" href="#__codelineno-0-27"></a><span class="n">profile_rule</span> <span class="o">=</span> <span class="n">PopulateRule</span><span class="p">(</span><span class="n">field_name</span><span class="o">=</span><span class="s2">&quot;profile&quot;</span><span class="p">,</span> <span class="n">collection_name</span><span class="o">=</span><span class="s2">&quot;profiles&quot;</span><span class="p">)</span>
<a id="__codelineno-0-28" name="__codelineno-0-28" href="#__codelineno-0-28"></a>
<a id="__codelineno-0-29" name="__codelineno-0-29" href="#__codelineno-0-29"></a>
<a id="__codelineno-0-30" name="__codelineno-0-30" href="#__codelineno-0-30"></a><span class="c1"># 3. Composed repository.</span>
<a id="__codelineno-0-31" name="__codelineno-0-31" href="#__codelineno-0-31"></a><span class="k">class</span><span class="w"> </span><span class="nc">CachedUserRepository</span><span class="p">(</span><span class="n">CachedBaseRepository</span><span class="p">[</span><span class="n">User</span><span class="p">]):</span>
<a id="__codelineno-0-32" name="__codelineno-0-32" href="#__codelineno-0-32"></a> <span class="k">def</span><span class="w"> </span><span class="fm">__init__</span><span class="p">(</span>
<a id="__codelineno-0-33" name="__codelineno-0-33" href="#__codelineno-0-33"></a> <span class="bp">self</span><span class="p">,</span>
<a id="__codelineno-0-34" name="__codelineno-0-34" href="#__codelineno-0-34"></a> <span class="n">cache_backend</span><span class="p">:</span> <span class="n">InMemoryCacheBackend</span><span class="p">,</span>
<a id="__codelineno-0-35" name="__codelineno-0-35" href="#__codelineno-0-35"></a> <span class="n">population_engine</span><span class="p">:</span> <span class="n">PopulationEngine</span><span class="p">,</span>
<a id="__codelineno-0-36" name="__codelineno-0-36" href="#__codelineno-0-36"></a> <span class="n">populate_rules</span><span class="p">:</span> <span class="nb">list</span><span class="p">[</span><span class="n">PopulateRule</span><span class="p">],</span>
<a id="__codelineno-0-37" name="__codelineno-0-37" href="#__codelineno-0-37"></a> <span class="n">config</span><span class="p">:</span> <span class="n">CacheConfig</span> <span class="o">|</span> <span class="kc">None</span> <span class="o">=</span> <span class="kc">None</span><span class="p">,</span>
<a id="__codelineno-0-38" name="__codelineno-0-38" href="#__codelineno-0-38"></a> <span class="p">):</span>
<a id="__codelineno-0-39" name="__codelineno-0-39" href="#__codelineno-0-39"></a> <span class="nb">super</span><span class="p">()</span><span class="o">.</span><span class="fm">__init__</span><span class="p">(</span><span class="s2">&quot;users&quot;</span><span class="p">,</span> <span class="n">User</span><span class="p">,</span> <span class="n">cache_backend</span><span class="p">,</span> <span class="n">config</span><span class="p">)</span>
<a id="__codelineno-0-40" name="__codelineno-0-40" href="#__codelineno-0-40"></a> <span class="bp">self</span><span class="o">.</span><span class="n">population_engine</span> <span class="o">=</span> <span class="n">population_engine</span>
<a id="__codelineno-0-41" name="__codelineno-0-41" href="#__codelineno-0-41"></a> <span class="bp">self</span><span class="o">.</span><span class="n">_populate_rules</span> <span class="o">=</span> <span class="n">populate_rules</span>
<a id="__codelineno-0-42" name="__codelineno-0-42" href="#__codelineno-0-42"></a>
<a id="__codelineno-0-43" name="__codelineno-0-43" href="#__codelineno-0-43"></a> <span class="k">async</span> <span class="k">def</span><span class="w"> </span><span class="nf">_populate</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">data</span><span class="p">:</span> <span class="nb">dict</span><span class="p">[</span><span class="nb">str</span><span class="p">,</span> <span class="n">Any</span><span class="p">])</span> <span class="o">-&gt;</span> <span class="nb">dict</span><span class="p">[</span><span class="nb">str</span><span class="p">,</span> <span class="n">Any</span><span class="p">]:</span>
<a id="__codelineno-0-44" name="__codelineno-0-44" href="#__codelineno-0-44"></a><span class="w"> </span><span class="sd">&quot;&quot;&quot;Dict-level resolution (mirrors PopulatingRepository._populate).&quot;&quot;&quot;</span>
<a id="__codelineno-0-45" name="__codelineno-0-45" href="#__codelineno-0-45"></a> <span class="k">for</span> <span class="n">rule</span> <span class="ow">in</span> <span class="bp">self</span><span class="o">.</span><span class="n">_populate_rules</span><span class="p">:</span>
<a id="__codelineno-0-46" name="__codelineno-0-46" href="#__codelineno-0-46"></a> <span class="n">ref</span> <span class="o">=</span> <span class="n">data</span><span class="o">.</span><span class="n">get</span><span class="p">(</span><span class="n">rule</span><span class="o">.</span><span class="n">field_name</span><span class="p">)</span>
<a id="__codelineno-0-47" name="__codelineno-0-47" href="#__codelineno-0-47"></a> <span class="k">if</span> <span class="n">ref</span> <span class="ow">is</span> <span class="kc">None</span><span class="p">:</span>
<a id="__codelineno-0-48" name="__codelineno-0-48" href="#__codelineno-0-48"></a> <span class="k">continue</span>
<a id="__codelineno-0-49" name="__codelineno-0-49" href="#__codelineno-0-49"></a> <span class="k">if</span> <span class="nb">isinstance</span><span class="p">(</span><span class="n">ref</span><span class="p">,</span> <span class="nb">str</span><span class="p">):</span> <span class="c1"># JSON round-trip turns ObjectId into hex str</span>
<a id="__codelineno-0-50" name="__codelineno-0-50" href="#__codelineno-0-50"></a> <span class="n">ref</span> <span class="o">=</span> <span class="n">ObjectId</span><span class="p">(</span><span class="n">ref</span><span class="p">)</span>
<a id="__codelineno-0-51" name="__codelineno-0-51" href="#__codelineno-0-51"></a> <span class="k">if</span> <span class="nb">isinstance</span><span class="p">(</span><span class="n">ref</span><span class="p">,</span> <span class="nb">list</span><span class="p">):</span>
<a id="__codelineno-0-52" name="__codelineno-0-52" href="#__codelineno-0-52"></a> <span class="n">resolved</span> <span class="o">=</span> <span class="p">[]</span>
<a id="__codelineno-0-53" name="__codelineno-0-53" href="#__codelineno-0-53"></a> <span class="k">for</span> <span class="n">item</span> <span class="ow">in</span> <span class="n">ref</span><span class="p">:</span>
<a id="__codelineno-0-54" name="__codelineno-0-54" href="#__codelineno-0-54"></a> <span class="n">item</span> <span class="o">=</span> <span class="n">ObjectId</span><span class="p">(</span><span class="n">item</span><span class="p">)</span> <span class="k">if</span> <span class="nb">isinstance</span><span class="p">(</span><span class="n">item</span><span class="p">,</span> <span class="nb">str</span><span class="p">)</span> <span class="k">else</span> <span class="n">item</span>
<a id="__codelineno-0-55" name="__codelineno-0-55" href="#__codelineno-0-55"></a> <span class="n">repo</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">population_engine</span><span class="o">.</span><span class="n">_repos</span><span class="o">.</span><span class="n">get</span><span class="p">(</span><span class="n">rule</span><span class="o">.</span><span class="n">collection_name</span><span class="p">)</span>
<a id="__codelineno-0-56" name="__codelineno-0-56" href="#__codelineno-0-56"></a> <span class="n">doc</span> <span class="o">=</span> <span class="k">await</span> <span class="n">repo</span><span class="o">.</span><span class="n">get_by_id</span><span class="p">(</span><span class="n">item</span><span class="p">)</span> <span class="k">if</span> <span class="n">repo</span> <span class="k">else</span> <span class="kc">None</span>
<a id="__codelineno-0-57" name="__codelineno-0-57" href="#__codelineno-0-57"></a> <span class="n">resolved</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="n">doc</span><span class="p">)</span>
<a id="__codelineno-0-58" name="__codelineno-0-58" href="#__codelineno-0-58"></a> <span class="n">data</span><span class="p">[</span><span class="n">rule</span><span class="o">.</span><span class="n">field_name</span><span class="p">]</span> <span class="o">=</span> <span class="n">resolved</span>
<a id="__codelineno-0-59" name="__codelineno-0-59" href="#__codelineno-0-59"></a> <span class="k">elif</span> <span class="nb">isinstance</span><span class="p">(</span><span class="n">ref</span><span class="p">,</span> <span class="n">ObjectId</span><span class="p">):</span>
<a id="__codelineno-0-60" name="__codelineno-0-60" href="#__codelineno-0-60"></a> <span class="n">repo</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">population_engine</span><span class="o">.</span><span class="n">_repos</span><span class="o">.</span><span class="n">get</span><span class="p">(</span><span class="n">rule</span><span class="o">.</span><span class="n">collection_name</span><span class="p">)</span>
<a id="__codelineno-0-61" name="__codelineno-0-61" href="#__codelineno-0-61"></a> <span class="n">data</span><span class="p">[</span><span class="n">rule</span><span class="o">.</span><span class="n">field_name</span><span class="p">]</span> <span class="o">=</span> <span class="k">await</span> <span class="n">repo</span><span class="o">.</span><span class="n">get_by_id</span><span class="p">(</span><span class="n">ref</span><span class="p">)</span> <span class="k">if</span> <span class="n">repo</span> <span class="k">else</span> <span class="kc">None</span>
<a id="__codelineno-0-62" name="__codelineno-0-62" href="#__codelineno-0-62"></a> <span class="k">return</span> <span class="n">data</span>
<a id="__codelineno-0-63" name="__codelineno-0-63" href="#__codelineno-0-63"></a>
<a id="__codelineno-0-64" name="__codelineno-0-64" href="#__codelineno-0-64"></a> <span class="k">async</span> <span class="k">def</span><span class="w"> </span><span class="nf">_depopulate</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">data</span><span class="p">:</span> <span class="n">User</span><span class="p">)</span> <span class="o">-&gt;</span> <span class="nb">dict</span><span class="p">[</span><span class="nb">str</span><span class="p">,</span> <span class="n">Any</span><span class="p">]:</span>
<a id="__codelineno-0-65" name="__codelineno-0-65" href="#__codelineno-0-65"></a><span class="w"> </span><span class="sd">&quot;&quot;&quot;Model -&gt; raw dict, FK fields collapsed to ObjectId (mirrors _depopulate).&quot;&quot;&quot;</span>
<a id="__codelineno-0-66" name="__codelineno-0-66" href="#__codelineno-0-66"></a> <span class="n">doc</span> <span class="o">=</span> <span class="n">data</span><span class="o">.</span><span class="n">model_dump</span><span class="p">(</span><span class="n">exclude</span><span class="o">=</span><span class="p">{</span><span class="s2">&quot;id&quot;</span><span class="p">},</span> <span class="n">exclude_none</span><span class="o">=</span><span class="kc">True</span><span class="p">)</span>
<a id="__codelineno-0-67" name="__codelineno-0-67" href="#__codelineno-0-67"></a> <span class="k">for</span> <span class="n">rule</span> <span class="ow">in</span> <span class="bp">self</span><span class="o">.</span><span class="n">_populate_rules</span><span class="p">:</span>
<a id="__codelineno-0-68" name="__codelineno-0-68" href="#__codelineno-0-68"></a> <span class="n">value</span> <span class="o">=</span> <span class="nb">getattr</span><span class="p">(</span><span class="n">data</span><span class="p">,</span> <span class="n">rule</span><span class="o">.</span><span class="n">field_name</span><span class="p">,</span> <span class="kc">None</span><span class="p">)</span>
<a id="__codelineno-0-69" name="__codelineno-0-69" href="#__codelineno-0-69"></a> <span class="k">if</span> <span class="nb">isinstance</span><span class="p">(</span><span class="n">value</span><span class="p">,</span> <span class="nb">list</span><span class="p">):</span>
<a id="__codelineno-0-70" name="__codelineno-0-70" href="#__codelineno-0-70"></a> <span class="n">doc</span><span class="p">[</span><span class="n">rule</span><span class="o">.</span><span class="n">field_name</span><span class="p">]</span> <span class="o">=</span> <span class="p">[</span><span class="n">item</span><span class="o">.</span><span class="n">id</span> <span class="k">for</span> <span class="n">item</span> <span class="ow">in</span> <span class="n">value</span><span class="p">]</span>
<a id="__codelineno-0-71" name="__codelineno-0-71" href="#__codelineno-0-71"></a> <span class="k">elif</span> <span class="nb">isinstance</span><span class="p">(</span><span class="n">value</span><span class="p">,</span> <span class="n">BaseDocument</span><span class="p">):</span>
<a id="__codelineno-0-72" name="__codelineno-0-72" href="#__codelineno-0-72"></a> <span class="n">doc</span><span class="p">[</span><span class="n">rule</span><span class="o">.</span><span class="n">field_name</span><span class="p">]</span> <span class="o">=</span> <span class="n">value</span><span class="o">.</span><span class="n">id</span>
<a id="__codelineno-0-73" name="__codelineno-0-73" href="#__codelineno-0-73"></a> <span class="k">return</span> <span class="n">doc</span>
<a id="__codelineno-0-74" name="__codelineno-0-74" href="#__codelineno-0-74"></a>
<a id="__codelineno-0-75" name="__codelineno-0-75" href="#__codelineno-0-75"></a> <span class="k">async</span> <span class="k">def</span><span class="w"> </span><span class="nf">create</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">data</span><span class="p">:</span> <span class="n">User</span><span class="p">)</span> <span class="o">-&gt;</span> <span class="n">User</span><span class="p">:</span>
<a id="__codelineno-0-76" name="__codelineno-0-76" href="#__codelineno-0-76"></a> <span class="n">doc</span> <span class="o">=</span> <span class="k">await</span> <span class="bp">self</span><span class="o">.</span><span class="n">_depopulate</span><span class="p">(</span><span class="n">data</span><span class="p">)</span> <span class="c1"># store FK refs as ObjectIds</span>
<a id="__codelineno-0-77" name="__codelineno-0-77" href="#__codelineno-0-77"></a> <span class="n">doc</span><span class="p">[</span><span class="s2">&quot;created_at&quot;</span><span class="p">]</span> <span class="o">=</span> <span class="n">datetime</span><span class="o">.</span><span class="n">utcnow</span><span class="p">()</span>
<a id="__codelineno-0-78" name="__codelineno-0-78" href="#__codelineno-0-78"></a> <span class="n">doc</span><span class="p">[</span><span class="s2">&quot;updated_at&quot;</span><span class="p">]</span> <span class="o">=</span> <span class="n">datetime</span><span class="o">.</span><span class="n">utcnow</span><span class="p">()</span>
<a id="__codelineno-0-79" name="__codelineno-0-79" href="#__codelineno-0-79"></a> <span class="n">result</span> <span class="o">=</span> <span class="k">await</span> <span class="bp">self</span><span class="o">.</span><span class="n">collection</span><span class="o">.</span><span class="n">insert_one</span><span class="p">(</span><span class="n">doc</span><span class="p">)</span>
<a id="__codelineno-0-80" name="__codelineno-0-80" href="#__codelineno-0-80"></a> <span class="n">doc</span><span class="p">[</span><span class="s2">&quot;_id&quot;</span><span class="p">]</span> <span class="o">=</span> <span class="n">result</span><span class="o">.</span><span class="n">inserted_id</span>
<a id="__codelineno-0-81" name="__codelineno-0-81" href="#__codelineno-0-81"></a> <span class="k">if</span> <span class="bp">self</span><span class="o">.</span><span class="n">_cache_config</span><span class="o">.</span><span class="n">enabled</span><span class="p">:</span>
<a id="__codelineno-0-82" name="__codelineno-0-82" href="#__codelineno-0-82"></a> <span class="k">await</span> <span class="bp">self</span><span class="o">.</span><span class="n">_cache</span><span class="o">.</span><span class="n">set</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">_cache_key</span><span class="p">(</span><span class="n">doc</span><span class="p">[</span><span class="s2">&quot;_id&quot;</span><span class="p">]),</span> <span class="n">encode_value</span><span class="p">(</span><span class="n">doc</span><span class="p">),</span> <span class="bp">self</span><span class="o">.</span><span class="n">_cache_config</span><span class="o">.</span><span class="n">default_ttl</span><span class="p">)</span>
<a id="__codelineno-0-83" name="__codelineno-0-83" href="#__codelineno-0-83"></a> <span class="k">return</span> <span class="bp">self</span><span class="o">.</span><span class="n">model</span><span class="p">(</span><span class="o">**</span><span class="k">await</span> <span class="bp">self</span><span class="o">.</span><span class="n">_populate</span><span class="p">(</span><span class="n">doc</span><span class="p">))</span>
<a id="__codelineno-0-84" name="__codelineno-0-84" href="#__codelineno-0-84"></a>
<a id="__codelineno-0-85" name="__codelineno-0-85" href="#__codelineno-0-85"></a> <span class="k">async</span> <span class="k">def</span><span class="w"> </span><span class="nf">update</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="nb">id</span><span class="p">:</span> <span class="n">Union</span><span class="p">[</span><span class="nb">str</span><span class="p">,</span> <span class="n">ObjectId</span><span class="p">],</span> <span class="n">data</span><span class="p">:</span> <span class="n">User</span><span class="p">)</span> <span class="o">-&gt;</span> <span class="n">User</span> <span class="o">|</span> <span class="kc">None</span><span class="p">:</span>
<a id="__codelineno-0-86" name="__codelineno-0-86" href="#__codelineno-0-86"></a> <span class="k">if</span> <span class="nb">isinstance</span><span class="p">(</span><span class="nb">id</span><span class="p">,</span> <span class="nb">str</span><span class="p">):</span>
<a id="__codelineno-0-87" name="__codelineno-0-87" href="#__codelineno-0-87"></a> <span class="nb">id</span> <span class="o">=</span> <span class="n">ObjectId</span><span class="p">(</span><span class="nb">id</span><span class="p">)</span>
<a id="__codelineno-0-88" name="__codelineno-0-88" href="#__codelineno-0-88"></a> <span class="n">doc</span> <span class="o">=</span> <span class="k">await</span> <span class="bp">self</span><span class="o">.</span><span class="n">_depopulate</span><span class="p">(</span><span class="n">data</span><span class="p">)</span> <span class="c1"># update() takes a full model here</span>
<a id="__codelineno-0-89" name="__codelineno-0-89" href="#__codelineno-0-89"></a> <span class="n">doc</span><span class="p">[</span><span class="s2">&quot;updated_at&quot;</span><span class="p">]</span> <span class="o">=</span> <span class="n">datetime</span><span class="o">.</span><span class="n">utcnow</span><span class="p">()</span>
<a id="__codelineno-0-90" name="__codelineno-0-90" href="#__codelineno-0-90"></a> <span class="n">result</span> <span class="o">=</span> <span class="k">await</span> <span class="bp">self</span><span class="o">.</span><span class="n">collection</span><span class="o">.</span><span class="n">find_one_and_update</span><span class="p">(</span>
<a id="__codelineno-0-91" name="__codelineno-0-91" href="#__codelineno-0-91"></a> <span class="p">{</span><span class="s2">&quot;_id&quot;</span><span class="p">:</span> <span class="nb">id</span><span class="p">},</span> <span class="p">{</span><span class="s2">&quot;$set&quot;</span><span class="p">:</span> <span class="n">doc</span><span class="p">},</span> <span class="n">return_document</span><span class="o">=</span><span class="kc">True</span>
<a id="__codelineno-0-92" name="__codelineno-0-92" href="#__codelineno-0-92"></a> <span class="p">)</span>
<a id="__codelineno-0-93" name="__codelineno-0-93" href="#__codelineno-0-93"></a> <span class="n">key</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">_cache_key</span><span class="p">(</span><span class="nb">id</span><span class="p">)</span>
<a id="__codelineno-0-94" name="__codelineno-0-94" href="#__codelineno-0-94"></a> <span class="k">if</span> <span class="n">result</span> <span class="ow">is</span> <span class="kc">None</span><span class="p">:</span>
<a id="__codelineno-0-95" name="__codelineno-0-95" href="#__codelineno-0-95"></a> <span class="k">if</span> <span class="bp">self</span><span class="o">.</span><span class="n">_cache_config</span><span class="o">.</span><span class="n">enabled</span><span class="p">:</span>
<a id="__codelineno-0-96" name="__codelineno-0-96" href="#__codelineno-0-96"></a> <span class="k">await</span> <span class="bp">self</span><span class="o">.</span><span class="n">_cache</span><span class="o">.</span><span class="n">delete</span><span class="p">(</span><span class="n">key</span><span class="p">)</span>
<a id="__codelineno-0-97" name="__codelineno-0-97" href="#__codelineno-0-97"></a> <span class="k">return</span> <span class="kc">None</span>
<a id="__codelineno-0-98" name="__codelineno-0-98" href="#__codelineno-0-98"></a> <span class="n">raw</span> <span class="o">=</span> <span class="nb">dict</span><span class="p">(</span><span class="n">result</span><span class="p">)</span>
<a id="__codelineno-0-99" name="__codelineno-0-99" href="#__codelineno-0-99"></a> <span class="k">if</span> <span class="bp">self</span><span class="o">.</span><span class="n">_cache_config</span><span class="o">.</span><span class="n">enabled</span><span class="p">:</span>
<a id="__codelineno-0-100" name="__codelineno-0-100" href="#__codelineno-0-100"></a> <span class="k">await</span> <span class="bp">self</span><span class="o">.</span><span class="n">_cache</span><span class="o">.</span><span class="n">set</span><span class="p">(</span><span class="n">key</span><span class="p">,</span> <span class="n">encode_value</span><span class="p">(</span><span class="n">raw</span><span class="p">),</span> <span class="bp">self</span><span class="o">.</span><span class="n">_cache_config</span><span class="o">.</span><span class="n">default_ttl</span><span class="p">)</span>
<a id="__codelineno-0-101" name="__codelineno-0-101" href="#__codelineno-0-101"></a> <span class="k">return</span> <span class="bp">self</span><span class="o">.</span><span class="n">model</span><span class="p">(</span><span class="o">**</span><span class="k">await</span> <span class="bp">self</span><span class="o">.</span><span class="n">_populate</span><span class="p">(</span><span class="n">raw</span><span class="p">))</span>
<a id="__codelineno-0-102" name="__codelineno-0-102" href="#__codelineno-0-102"></a>
<a id="__codelineno-0-103" name="__codelineno-0-103" href="#__codelineno-0-103"></a> <span class="k">async</span> <span class="k">def</span><span class="w"> </span><span class="nf">get_by_id</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="nb">id</span><span class="p">:</span> <span class="n">Union</span><span class="p">[</span><span class="nb">str</span><span class="p">,</span> <span class="n">ObjectId</span><span class="p">])</span> <span class="o">-&gt;</span> <span class="n">User</span> <span class="o">|</span> <span class="kc">None</span><span class="p">:</span>
<a id="__codelineno-0-104" name="__codelineno-0-104" href="#__codelineno-0-104"></a> <span class="k">if</span> <span class="ow">not</span> <span class="bp">self</span><span class="o">.</span><span class="n">_cache_config</span><span class="o">.</span><span class="n">enabled</span><span class="p">:</span>
<a id="__codelineno-0-105" name="__codelineno-0-105" href="#__codelineno-0-105"></a> <span class="c1"># No cache: still a raw fetch + populate — the base get_by_id()</span>
<a id="__codelineno-0-106" name="__codelineno-0-106" href="#__codelineno-0-106"></a> <span class="c1"># would build a typed model from the raw doc and fail on FK fields.</span>
<a id="__codelineno-0-107" name="__codelineno-0-107" href="#__codelineno-0-107"></a> <span class="n">raw</span> <span class="o">=</span> <span class="k">await</span> <span class="bp">self</span><span class="o">.</span><span class="n">collection</span><span class="o">.</span><span class="n">find_one</span><span class="p">({</span><span class="s2">&quot;_id&quot;</span><span class="p">:</span> <span class="n">ObjectId</span><span class="p">(</span><span class="nb">id</span><span class="p">)</span> <span class="k">if</span> <span class="nb">isinstance</span><span class="p">(</span><span class="nb">id</span><span class="p">,</span> <span class="nb">str</span><span class="p">)</span> <span class="k">else</span> <span class="nb">id</span><span class="p">})</span>
<a id="__codelineno-0-108" name="__codelineno-0-108" href="#__codelineno-0-108"></a> <span class="k">return</span> <span class="bp">self</span><span class="o">.</span><span class="n">model</span><span class="p">(</span><span class="o">**</span><span class="k">await</span> <span class="bp">self</span><span class="o">.</span><span class="n">_populate</span><span class="p">(</span><span class="n">raw</span><span class="p">))</span> <span class="k">if</span> <span class="n">raw</span> <span class="k">else</span> <span class="kc">None</span>
<a id="__codelineno-0-109" name="__codelineno-0-109" href="#__codelineno-0-109"></a>
<a id="__codelineno-0-110" name="__codelineno-0-110" href="#__codelineno-0-110"></a> <span class="n">cached</span> <span class="o">=</span> <span class="k">await</span> <span class="bp">self</span><span class="o">.</span><span class="n">_cache</span><span class="o">.</span><span class="n">get</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">_cache_key</span><span class="p">(</span><span class="nb">id</span><span class="p">))</span>
<a id="__codelineno-0-111" name="__codelineno-0-111" href="#__codelineno-0-111"></a> <span class="k">if</span> <span class="n">cached</span> <span class="ow">is</span> <span class="ow">not</span> <span class="kc">None</span><span class="p">:</span>
<a id="__codelineno-0-112" name="__codelineno-0-112" href="#__codelineno-0-112"></a> <span class="c1"># Cache hit: decode the RAW doc, then populate before returning.</span>
<a id="__codelineno-0-113" name="__codelineno-0-113" href="#__codelineno-0-113"></a> <span class="n">data</span> <span class="o">=</span> <span class="k">await</span> <span class="bp">self</span><span class="o">.</span><span class="n">_populate</span><span class="p">(</span><span class="n">decode_value</span><span class="p">(</span><span class="n">cached</span><span class="p">))</span>
<a id="__codelineno-0-114" name="__codelineno-0-114" href="#__codelineno-0-114"></a> <span class="k">return</span> <span class="bp">self</span><span class="o">.</span><span class="n">model</span><span class="p">(</span><span class="o">**</span><span class="n">data</span><span class="p">)</span>
<a id="__codelineno-0-115" name="__codelineno-0-115" href="#__codelineno-0-115"></a>
<a id="__codelineno-0-116" name="__codelineno-0-116" href="#__codelineno-0-116"></a> <span class="c1"># Cache miss: one RAW DB read. The base get_by_id() would rebuild the</span>
<a id="__codelineno-0-117" name="__codelineno-0-117" href="#__codelineno-0-117"></a> <span class="c1"># model from the raw doc, which fails for model-typed FK fields —</span>
<a id="__codelineno-0-118" name="__codelineno-0-118" href="#__codelineno-0-118"></a> <span class="c1"># so fetch the raw dict, cache it, and populate before materializing.</span>
<a id="__codelineno-0-119" name="__codelineno-0-119" href="#__codelineno-0-119"></a> <span class="n">key</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">_cache_key</span><span class="p">(</span><span class="nb">id</span><span class="p">)</span>
<a id="__codelineno-0-120" name="__codelineno-0-120" href="#__codelineno-0-120"></a> <span class="n">raw</span> <span class="o">=</span> <span class="k">await</span> <span class="bp">self</span><span class="o">.</span><span class="n">collection</span><span class="o">.</span><span class="n">find_one</span><span class="p">({</span><span class="s2">&quot;_id&quot;</span><span class="p">:</span> <span class="n">ObjectId</span><span class="p">(</span><span class="nb">id</span><span class="p">)</span> <span class="k">if</span> <span class="nb">isinstance</span><span class="p">(</span><span class="nb">id</span><span class="p">,</span> <span class="nb">str</span><span class="p">)</span> <span class="k">else</span> <span class="nb">id</span><span class="p">})</span>
<a id="__codelineno-0-121" name="__codelineno-0-121" href="#__codelineno-0-121"></a> <span class="k">if</span> <span class="n">raw</span> <span class="ow">is</span> <span class="kc">None</span><span class="p">:</span>
<a id="__codelineno-0-122" name="__codelineno-0-122" href="#__codelineno-0-122"></a> <span class="k">return</span> <span class="kc">None</span>
<a id="__codelineno-0-123" name="__codelineno-0-123" href="#__codelineno-0-123"></a> <span class="k">await</span> <span class="bp">self</span><span class="o">.</span><span class="n">_cache</span><span class="o">.</span><span class="n">set</span><span class="p">(</span><span class="n">key</span><span class="p">,</span> <span class="n">encode_value</span><span class="p">(</span><span class="n">raw</span><span class="p">),</span> <span class="bp">self</span><span class="o">.</span><span class="n">_cache_config</span><span class="o">.</span><span class="n">default_ttl</span><span class="p">)</span>
<a id="__codelineno-0-124" name="__codelineno-0-124" href="#__codelineno-0-124"></a> <span class="k">return</span> <span class="bp">self</span><span class="o">.</span><span class="n">model</span><span class="p">(</span><span class="o">**</span><span class="k">await</span> <span class="bp">self</span><span class="o">.</span><span class="n">_populate</span><span class="p">(</span><span class="n">raw</span><span class="p">))</span>
<a id="__codelineno-0-125" name="__codelineno-0-125" href="#__codelineno-0-125"></a>
<a id="__codelineno-0-126" name="__codelineno-0-126" href="#__codelineno-0-126"></a>
<a id="__codelineno-0-127" name="__codelineno-0-127" href="#__codelineno-0-127"></a><span class="c1"># 4. Wire-up — one backend for both the repo and the registry lifecycle.</span>
<a id="__codelineno-0-128" name="__codelineno-0-128" href="#__codelineno-0-128"></a><span class="n">cache</span> <span class="o">=</span> <span class="n">InMemoryCacheBackend</span><span class="p">(</span><span class="n">max_entries</span><span class="o">=</span><span class="mi">20_000</span><span class="p">,</span> <span class="n">default_ttl</span><span class="o">=</span><span class="mi">600</span><span class="p">)</span>
<a id="__codelineno-0-129" name="__codelineno-0-129" href="#__codelineno-0-129"></a><span class="n">ModelRegistry</span><span class="o">.</span><span class="n">set_cache_backend</span><span class="p">(</span><span class="n">cache</span><span class="p">)</span>
<a id="__codelineno-0-130" name="__codelineno-0-130" href="#__codelineno-0-130"></a>
<a id="__codelineno-0-131" name="__codelineno-0-131" href="#__codelineno-0-131"></a>
<a id="__codelineno-0-132" name="__codelineno-0-132" href="#__codelineno-0-132"></a><span class="nd">@asynccontextmanager</span>
<a id="__codelineno-0-133" name="__codelineno-0-133" href="#__codelineno-0-133"></a><span class="k">async</span> <span class="k">def</span><span class="w"> </span><span class="nf">lifespan</span><span class="p">(</span><span class="n">_app</span><span class="p">:</span> <span class="n">FastAPI</span><span class="p">):</span>
<a id="__codelineno-0-134" name="__codelineno-0-134" href="#__codelineno-0-134"></a> <span class="k">async</span> <span class="k">with</span> <span class="n">MongoConnectionManager</span><span class="o">.</span><span class="n">lifespan</span><span class="p">(</span>
<a id="__codelineno-0-135" name="__codelineno-0-135" href="#__codelineno-0-135"></a> <span class="n">uri</span><span class="o">=</span><span class="s2">&quot;mongodb://localhost:27017&quot;</span><span class="p">,</span> <span class="n">db_name</span><span class="o">=</span><span class="s2">&quot;app_db&quot;</span>
<a id="__codelineno-0-136" name="__codelineno-0-136" href="#__codelineno-0-136"></a> <span class="p">):</span>
<a id="__codelineno-0-137" name="__codelineno-0-137" href="#__codelineno-0-137"></a> <span class="c1"># Register the repositories the engine resolves refs against:</span>
<a id="__codelineno-0-138" name="__codelineno-0-138" href="#__codelineno-0-138"></a> <span class="c1"># engine.register_repo(&quot;profiles&quot;, ProfileRepo())</span>
<a id="__codelineno-0-139" name="__codelineno-0-139" href="#__codelineno-0-139"></a> <span class="n">engine</span><span class="o">.</span><span class="n">register_repo</span><span class="p">(</span><span class="s2">&quot;users&quot;</span><span class="p">,</span> <span class="n">CachedUserRepository</span><span class="p">(</span><span class="n">cache</span><span class="p">,</span> <span class="n">engine</span><span class="p">,</span> <span class="p">[</span><span class="n">profile_rule</span><span class="p">]))</span>
<a id="__codelineno-0-140" name="__codelineno-0-140" href="#__codelineno-0-140"></a> <span class="k">await</span> <span class="n">ModelRegistry</span><span class="o">.</span><span class="n">initialize_all</span><span class="p">()</span>
<a id="__codelineno-0-141" name="__codelineno-0-141" href="#__codelineno-0-141"></a> <span class="k">await</span> <span class="n">ModelRegistry</span><span class="o">.</span><span class="n">initialize_cache</span><span class="p">()</span>
<a id="__codelineno-0-142" name="__codelineno-0-142" href="#__codelineno-0-142"></a> <span class="k">yield</span>
<a id="__codelineno-0-143" name="__codelineno-0-143" href="#__codelineno-0-143"></a> <span class="k">await</span> <span class="n">ModelRegistry</span><span class="o">.</span><span class="n">shutdown_cache</span><span class="p">()</span>
<a id="__codelineno-0-144" name="__codelineno-0-144" href="#__codelineno-0-144"></a>
<a id="__codelineno-0-145" name="__codelineno-0-145" href="#__codelineno-0-145"></a>
<a id="__codelineno-0-146" name="__codelineno-0-146" href="#__codelineno-0-146"></a><span class="n">app</span> <span class="o">=</span> <span class="n">FastAPI</span><span class="p">(</span><span class="n">lifespan</span><span class="o">=</span><span class="n">lifespan</span><span class="p">)</span>
</code></pre></div>
<blockquote>
<p><strong>Note:</strong> replace the placeholder <code>engine.register_repo("profiles", ...)</code> line with a repository for <code>Profile</code>. A matching <code>CachedBaseRepository[Profile]("profiles", Profile, cache)</code> keeps profile reads cached too — e.g. <code>engine.register_repo("profiles", CachedBaseRepository[Profile]("profiles", Profile, cache))</code>.</p>
</blockquote>
<hr />
<h2 id="tips">💡 Tips</h2>
<ul>
<li><strong>Cache the raw doc, populate at read time.</strong> Ref fields stay <code>ObjectId</code>s in the cache; JSON round-trips them to hex strings, so <code>_populate</code> must normalize <code>str → ObjectId</code> (shown above).</li>
<li><strong><code>update</code>/<code>create</code> above are model-based</strong> (<code>update(id, User)</code>), mirroring <code>PopulatingRepository</code> — they depopulate before writing and cache a <strong>raw</strong> snapshot. The plain <code>CachedBaseRepository.update(id, dict)</code> and <code>delete(id)</code> keep working and invalidate the same key.</li>
<li>Set a sensible <code>default_ttl</code> — cached <code>User</code> entries resolve <code>Profile</code> on each read, so profile edits show up within the TTL (or call <code>invalidate_cache(user_id)</code> explicitly).</li>
<li>To reuse this compose logic across many collections, extract the <code>_populate</code> / <code>_depopulate</code> helpers plus the <code>create</code> / <code>update</code> / <code>get_by_id</code> overrides into a mixin and parameterize the rules per subclass.</li>
</ul>
<hr />
<h2 id="related">Related</h2>
<ul>
<li><a href="../07_caching/">07 – Caching</a> · <a href="../08_population/">08 – Population</a> · <a href="../15_populating_repository_wiring/">15 – PopulatingRepository internals</a> · <a href="../16_cached_repository_intricacies/">16 – Cached repository intricacies</a></li>
</ul>
</article>
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