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<h1 id="use-case-9-nested-document-population-circular-ref-handling">Use Case 9: Nested Document Population &amp; Circular-Ref Handling</h1>
<p><strong>Scenario:</strong> An <code>Author</code> has <code>books</code> (a list of references), each <code>Book</code> references a <code>publisher</code>, and an <code>Author</code> may reference a <code>mentor</code> — which is <em>another</em> <code>Author</code> (a potential cycle).</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>nested_rules</code></td>
<td>Populates deeper levels: resolve <code>books</code> on <code>Author</code>, then <code>publisher</code> inside each <code>Book</code>.</td>
</tr>
<tr>
<td><code>max_depth</code></td>
<td>Per-rule recursion bound — the safety net for cyclic graphs.</td>
</tr>
<tr>
<td><code>global_max_depth</code></td>
<td><code>PopulationEngine(repos, global_max_depth=10)</code> global cap.</td>
</tr>
<tr>
<td><code>CircularReferenceError</code></td>
<td>Raised when a <code>(Class, id)</code> pair is revisited; carries the visited path.</td>
</tr>
</tbody>
</table>
<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>
<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">bson</span><span class="w"> </span><span class="kn">import</span> <span class="n">ObjectId</span>
<a id="__codelineno-0-4" name="__codelineno-0-4" href="#__codelineno-0-4"></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><span class="p">,</span> <span class="n">HTTPException</span>
<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">mongo_ops</span><span class="w"> </span><span class="kn">import</span> <span class="n">BaseDocument</span><span class="p">,</span> <span class="n">ModelRegistry</span><span class="p">,</span> <span class="n">MongoConnectionManager</span><span class="p">,</span> <span class="n">PopulatingRepository</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">mongo_ops.cache</span><span class="w"> </span><span class="kn">import</span> <span class="n">CircularReferenceError</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.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-0-8" name="__codelineno-0-8" href="#__codelineno-0-8"></a>
<a id="__codelineno-0-9" name="__codelineno-0-9" href="#__codelineno-0-9"></a>
<a id="__codelineno-0-10" name="__codelineno-0-10" href="#__codelineno-0-10"></a><span class="c1"># 1. Models — each ref field holds ObjectId(s) in the DB and becomes model(s) in memory.</span>
<a id="__codelineno-0-11" name="__codelineno-0-11" href="#__codelineno-0-11"></a><span class="k">class</span><span class="w"> </span><span class="nc">Publisher</span><span class="p">(</span><span class="n">BaseDocument</span><span class="p">):</span>
<a id="__codelineno-0-12" name="__codelineno-0-12" href="#__codelineno-0-12"></a> <span class="n">name</span><span class="p">:</span> <span class="nb">str</span> <span class="o">=</span> <span class="s2">&quot;&quot;</span>
<a id="__codelineno-0-13" name="__codelineno-0-13" href="#__codelineno-0-13"></a> <span class="n">country</span><span class="p">:</span> <span class="nb">str</span> <span class="o">=</span> <span class="s2">&quot;&quot;</span>
<a id="__codelineno-0-14" name="__codelineno-0-14" href="#__codelineno-0-14"></a>
<a id="__codelineno-0-15" name="__codelineno-0-15" href="#__codelineno-0-15"></a>
<a id="__codelineno-0-16" name="__codelineno-0-16" href="#__codelineno-0-16"></a><span class="k">class</span><span class="w"> </span><span class="nc">Book</span><span class="p">(</span><span class="n">BaseDocument</span><span class="p">):</span>
<a id="__codelineno-0-17" name="__codelineno-0-17" href="#__codelineno-0-17"></a> <span class="n">title</span><span class="p">:</span> <span class="nb">str</span> <span class="o">=</span> <span class="s2">&quot;&quot;</span>
<a id="__codelineno-0-18" name="__codelineno-0-18" href="#__codelineno-0-18"></a> <span class="n">publisher</span><span class="p">:</span> <span class="n">Publisher</span> <span class="o">|</span> <span class="kc">None</span> <span class="o">=</span> <span class="kc">None</span> <span class="c1"># ObjectId in DB, Publisher in memory</span>
<a id="__codelineno-0-19" name="__codelineno-0-19" href="#__codelineno-0-19"></a>
<a id="__codelineno-0-20" name="__codelineno-0-20" href="#__codelineno-0-20"></a>
<a id="__codelineno-0-21" name="__codelineno-0-21" href="#__codelineno-0-21"></a><span class="k">class</span><span class="w"> </span><span class="nc">Author</span><span class="p">(</span><span class="n">BaseDocument</span><span class="p">):</span>
<a id="__codelineno-0-22" name="__codelineno-0-22" href="#__codelineno-0-22"></a> <span class="n">name</span><span class="p">:</span> <span class="nb">str</span> <span class="o">=</span> <span class="s2">&quot;&quot;</span>
<a id="__codelineno-0-23" name="__codelineno-0-23" href="#__codelineno-0-23"></a> <span class="n">books</span><span class="p">:</span> <span class="nb">list</span><span class="p">[</span><span class="n">Book</span><span class="p">]</span> <span class="o">|</span> <span class="kc">None</span> <span class="o">=</span> <span class="kc">None</span> <span class="c1"># list[ObjectId] in DB, list[Book] in memory</span>
<a id="__codelineno-0-24" name="__codelineno-0-24" href="#__codelineno-0-24"></a> <span class="n">mentor</span><span class="p">:</span> <span class="s2">&quot;Author&quot;</span> <span class="o">|</span> <span class="kc">None</span> <span class="o">=</span> <span class="kc">None</span> <span class="c1"># self-reference — potential cycle</span>
<a id="__codelineno-0-25" name="__codelineno-0-25" href="#__codelineno-0-25"></a>
<a id="__codelineno-0-26" name="__codelineno-0-26" href="#__codelineno-0-26"></a>
<a id="__codelineno-0-27" name="__codelineno-0-27" href="#__codelineno-0-27"></a><span class="c1"># 2. Engine + nested rules.</span>
<a id="__codelineno-0-28" name="__codelineno-0-28" href="#__codelineno-0-28"></a><span class="n">engine</span> <span class="o">=</span> <span class="n">PopulationEngine</span><span class="p">({})</span>
<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="n">publisher_rule</span> <span class="o">=</span> <span class="n">PopulateRule</span><span class="p">(</span>
<a id="__codelineno-0-31" name="__codelineno-0-31" href="#__codelineno-0-31"></a> <span class="n">field_name</span><span class="o">=</span><span class="s2">&quot;publisher&quot;</span><span class="p">,</span>
<a id="__codelineno-0-32" name="__codelineno-0-32" href="#__codelineno-0-32"></a> <span class="n">collection_name</span><span class="o">=</span><span class="s2">&quot;publishers&quot;</span><span class="p">,</span>
<a id="__codelineno-0-33" name="__codelineno-0-33" href="#__codelineno-0-33"></a><span class="p">)</span>
<a id="__codelineno-0-34" name="__codelineno-0-34" href="#__codelineno-0-34"></a>
<a id="__codelineno-0-35" name="__codelineno-0-35" href="#__codelineno-0-35"></a><span class="n">book_rule</span> <span class="o">=</span> <span class="n">PopulateRule</span><span class="p">(</span>
<a id="__codelineno-0-36" name="__codelineno-0-36" href="#__codelineno-0-36"></a> <span class="n">field_name</span><span class="o">=</span><span class="s2">&quot;books&quot;</span><span class="p">,</span>
<a id="__codelineno-0-37" name="__codelineno-0-37" href="#__codelineno-0-37"></a> <span class="n">collection_name</span><span class="o">=</span><span class="s2">&quot;books&quot;</span><span class="p">,</span>
<a id="__codelineno-0-38" name="__codelineno-0-38" href="#__codelineno-0-38"></a> <span class="n">nested_rules</span><span class="o">=</span><span class="p">[</span><span class="n">publisher_rule</span><span class="p">],</span> <span class="c1"># fetch each book, then its publisher</span>
<a id="__codelineno-0-39" name="__codelineno-0-39" href="#__codelineno-0-39"></a> <span class="n">max_depth</span><span class="o">=</span><span class="mi">3</span><span class="p">,</span>
<a id="__codelineno-0-40" name="__codelineno-0-40" href="#__codelineno-0-40"></a><span class="p">)</span>
<a id="__codelineno-0-41" name="__codelineno-0-41" href="#__codelineno-0-41"></a>
<a id="__codelineno-0-42" name="__codelineno-0-42" href="#__codelineno-0-42"></a><span class="n">mentor_rule</span> <span class="o">=</span> <span class="n">PopulateRule</span><span class="p">(</span>
<a id="__codelineno-0-43" name="__codelineno-0-43" href="#__codelineno-0-43"></a> <span class="n">field_name</span><span class="o">=</span><span class="s2">&quot;mentor&quot;</span><span class="p">,</span>
<a id="__codelineno-0-44" name="__codelineno-0-44" href="#__codelineno-0-44"></a> <span class="n">collection_name</span><span class="o">=</span><span class="s2">&quot;authors&quot;</span><span class="p">,</span>
<a id="__codelineno-0-45" name="__codelineno-0-45" href="#__codelineno-0-45"></a> <span class="n">max_depth</span><span class="o">=</span><span class="mi">2</span><span class="p">,</span> <span class="c1"># stops mentor chains early — also avoids unbounded cycles</span>
<a id="__codelineno-0-46" name="__codelineno-0-46" href="#__codelineno-0-46"></a><span class="p">)</span>
<a id="__codelineno-0-47" name="__codelineno-0-47" href="#__codelineno-0-47"></a>
<a id="__codelineno-0-48" name="__codelineno-0-48" href="#__codelineno-0-48"></a>
<a id="__codelineno-0-49" name="__codelineno-0-49" href="#__codelineno-0-49"></a><span class="k">class</span><span class="w"> </span><span class="nc">AuthorRepository</span><span class="p">(</span><span class="n">PopulatingRepository</span><span class="p">[</span><span class="n">Author</span><span class="p">]):</span>
<a id="__codelineno-0-50" name="__codelineno-0-50" href="#__codelineno-0-50"></a> <span class="k">def</span><span class="w"> </span><span class="fm">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<a id="__codelineno-0-51" name="__codelineno-0-51" href="#__codelineno-0-51"></a> <span class="nb">super</span><span class="p">()</span><span class="o">.</span><span class="fm">__init__</span><span class="p">(</span>
<a id="__codelineno-0-52" name="__codelineno-0-52" href="#__codelineno-0-52"></a> <span class="n">collection_name</span><span class="o">=</span><span class="s2">&quot;authors&quot;</span><span class="p">,</span>
<a id="__codelineno-0-53" name="__codelineno-0-53" href="#__codelineno-0-53"></a> <span class="n">model</span><span class="o">=</span><span class="n">Author</span><span class="p">,</span>
<a id="__codelineno-0-54" name="__codelineno-0-54" href="#__codelineno-0-54"></a> <span class="n">population_engine</span><span class="o">=</span><span class="n">engine</span><span class="p">,</span>
<a id="__codelineno-0-55" name="__codelineno-0-55" href="#__codelineno-0-55"></a> <span class="n">populate_rules</span><span class="o">=</span><span class="p">[</span><span class="n">book_rule</span><span class="p">,</span> <span class="n">mentor_rule</span><span class="p">],</span>
<a id="__codelineno-0-56" name="__codelineno-0-56" href="#__codelineno-0-56"></a> <span class="p">)</span>
<a id="__codelineno-0-57" name="__codelineno-0-57" href="#__codelineno-0-57"></a>
<a id="__codelineno-0-58" name="__codelineno-0-58" href="#__codelineno-0-58"></a>
<a id="__codelineno-0-59" name="__codelineno-0-59" href="#__codelineno-0-59"></a><span class="nd">@asynccontextmanager</span>
<a id="__codelineno-0-60" name="__codelineno-0-60" href="#__codelineno-0-60"></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-0-61" name="__codelineno-0-61" href="#__codelineno-0-61"></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-0-62" name="__codelineno-0-62" href="#__codelineno-0-62"></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;library&quot;</span>
<a id="__codelineno-0-63" name="__codelineno-0-63" href="#__codelineno-0-63"></a> <span class="p">):</span>
<a id="__codelineno-0-64" name="__codelineno-0-64" href="#__codelineno-0-64"></a> <span class="n">engine</span><span class="o">.</span><span class="n">register_repo</span><span class="p">(</span><span class="s2">&quot;publishers&quot;</span><span class="p">,</span> <span class="n">PopulatingRepository</span><span class="p">[</span><span class="n">Publisher</span><span class="p">](</span><span class="s2">&quot;publishers&quot;</span><span class="p">,</span> <span class="n">Publisher</span><span class="p">))</span>
<a id="__codelineno-0-65" name="__codelineno-0-65" href="#__codelineno-0-65"></a> <span class="n">engine</span><span class="o">.</span><span class="n">register_repo</span><span class="p">(</span><span class="s2">&quot;books&quot;</span><span class="p">,</span> <span class="n">PopulatingRepository</span><span class="p">[</span><span class="n">Book</span><span class="p">](</span><span class="s2">&quot;books&quot;</span><span class="p">,</span> <span class="n">Book</span><span class="p">))</span>
<a id="__codelineno-0-66" name="__codelineno-0-66" href="#__codelineno-0-66"></a> <span class="n">engine</span><span class="o">.</span><span class="n">register_repo</span><span class="p">(</span><span class="s2">&quot;authors&quot;</span><span class="p">,</span> <span class="n">AuthorRepository</span><span class="p">())</span>
<a id="__codelineno-0-67" name="__codelineno-0-67" href="#__codelineno-0-67"></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-0-68" name="__codelineno-0-68" href="#__codelineno-0-68"></a> <span class="k">yield</span>
<a id="__codelineno-0-69" name="__codelineno-0-69" href="#__codelineno-0-69"></a>
<a id="__codelineno-0-70" name="__codelineno-0-70" href="#__codelineno-0-70"></a>
<a id="__codelineno-0-71" name="__codelineno-0-71" href="#__codelineno-0-71"></a><span class="n">app</span> <span class="o">=</span> <span class="n">FastAPI</span><span class="p">()</span>
<a id="__codelineno-0-72" name="__codelineno-0-72" href="#__codelineno-0-72"></a>
<a id="__codelineno-0-73" name="__codelineno-0-73" href="#__codelineno-0-73"></a>
<a id="__codelineno-0-74" name="__codelineno-0-74" href="#__codelineno-0-74"></a><span class="nd">@app</span><span class="o">.</span><span class="n">get</span><span class="p">(</span><span class="s2">&quot;/authors/</span><span class="si">{author_id}</span><span class="s2">&quot;</span><span class="p">)</span>
<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">get_author</span><span class="p">(</span><span class="n">author_id</span><span class="p">:</span> <span class="nb">str</span><span class="p">):</span>
<a id="__codelineno-0-76" name="__codelineno-0-76" href="#__codelineno-0-76"></a> <span class="k">try</span><span class="p">:</span>
<a id="__codelineno-0-77" name="__codelineno-0-77" href="#__codelineno-0-77"></a> <span class="n">author</span> <span class="o">=</span> <span class="k">await</span> <span class="n">AuthorRepository</span><span class="p">()</span><span class="o">.</span><span class="n">get_by_id</span><span class="p">(</span><span class="n">author_id</span><span class="p">)</span>
<a id="__codelineno-0-78" name="__codelineno-0-78" href="#__codelineno-0-78"></a> <span class="k">except</span> <span class="n">CircularReferenceError</span> <span class="k">as</span> <span class="n">exc</span><span class="p">:</span>
<a id="__codelineno-0-79" name="__codelineno-0-79" href="#__codelineno-0-79"></a> <span class="k">raise</span> <span class="n">HTTPException</span><span class="p">(</span><span class="n">status_code</span><span class="o">=</span><span class="mi">409</span><span class="p">,</span> <span class="n">detail</span><span class="o">=</span><span class="sa">f</span><span class="s2">&quot;Circular reference: </span><span class="si">{</span><span class="n">exc</span><span class="si">}</span><span class="s2">&quot;</span><span class="p">)</span>
<a id="__codelineno-0-80" name="__codelineno-0-80" href="#__codelineno-0-80"></a> <span class="k">if</span> <span class="ow">not</span> <span class="n">author</span><span class="p">:</span>
<a id="__codelineno-0-81" name="__codelineno-0-81" href="#__codelineno-0-81"></a> <span class="k">raise</span> <span class="n">HTTPException</span><span class="p">(</span><span class="n">status_code</span><span class="o">=</span><span class="mi">404</span><span class="p">,</span> <span class="n">detail</span><span class="o">=</span><span class="s2">&quot;Author not found&quot;</span><span class="p">)</span>
<a id="__codelineno-0-82" name="__codelineno-0-82" href="#__codelineno-0-82"></a> <span class="k">return</span> <span class="n">author</span>
</code></pre></div>
<hr />
<h2 id="tips">💡 Tips</h2>
<ul>
<li><code>CircularReferenceError</code> is a <code>ValueError</code> subtype exposing <code>.collection</code>, <code>.doc_id</code>, and the visited <code>.path</code> — use it in error responses and logging.</li>
<li>Keep <code>max_depth</code> conservative (2–3) for most graphs; combine with the global <code>global_max_depth=10</code> default.</li>
<li>List refs depopulate back to <code>list[ObjectId]</code> on <code>create</code>/<code>update</code>, mirroring the scalar case.</li>
</ul>
<hr />
<h2 id="related">Related</h2>
<ul>
<li><a href="../08_population/">08 – Population</a> · <a href="../15_populating_repository_wiring/">15 – PopulatingRepository internals</a> · <a href="../17_populate_depopulate_examples/">17 – Populate &amp; depopulate examples</a> · <a href="../10_cache_and_population/">10 – Cache + population</a> · <a href="../../06_error_handling/">Error Handling</a></li>
</ul>
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