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<title>Using <span class="caps">DISTINCT</span> in Parent-Child Relationships - Alexis Métaireau</title>
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<h1 class="post-title">Using <span class="caps">DISTINCT</span> in Parent-Child Relationships</h1>
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<p><em>How to get parent and most-recent child in a one-to-many relationship</em></p>
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<time datetime="2023-10-18T00:00:00+02:00">18 octobre 2023</time>
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<p>Let’s say you have a model defined like this, with a Parent and a Child table:</p>
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<div class="highlight"><pre><span></span><code><span class="k">class</span> <span class="nc">Parent</span><span class="p">(</span><span class="n">Base</span><span class="p">):</span>
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<span class="n">__tablename__</span> <span class="o">=</span> <span class="s2">"parent"</span>
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<span class="nb">id</span><span class="p">:</span> <span class="n">Mapped</span><span class="p">[</span><span class="nb">int</span><span class="p">]</span> <span class="o">=</span> <span class="n">mapped_column</span><span class="p">(</span><span class="n">primary_key</span><span class="o">=</span><span class="kc">True</span><span class="p">)</span>
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<span class="n">childs</span><span class="p">:</span> <span class="n">Mapped</span><span class="p">[</span><span class="n">List</span><span class="p">[</span><span class="s2">"Child"</span><span class="p">]]</span> <span class="o">=</span> <span class="n">relationship</span><span class="p">(</span><span class="n">back_populates</span><span class="o">=</span><span class="s2">"parent"</span><span class="p">)</span>
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<span class="k">class</span> <span class="nc">Child</span><span class="p">(</span><span class="n">Base</span><span class="p">):</span>
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<span class="n">__tablename__</span> <span class="o">=</span> <span class="s2">"child"</span>
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<span class="nb">id</span><span class="p">:</span> <span class="n">Mapped</span><span class="p">[</span><span class="nb">int</span><span class="p">]</span> <span class="o">=</span> <span class="n">mapped_column</span><span class="p">(</span><span class="n">primary_key</span><span class="o">=</span><span class="kc">True</span><span class="p">)</span>
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<span class="n">parent_id</span><span class="p">:</span> <span class="n">Mapped</span><span class="p">[</span><span class="nb">int</span><span class="p">]</span> <span class="o">=</span> <span class="n">mapped_column</span><span class="p">(</span><span class="n">ForeignKey</span><span class="p">(</span><span class="s2">"parent.id"</span><span class="p">))</span>
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<span class="n">parent</span><span class="p">:</span> <span class="n">Mapped</span><span class="p">[</span><span class="s2">"Parent"</span><span class="p">]</span> <span class="o">=</span> <span class="n">relationship</span><span class="p">(</span><span class="n">back_populates</span><span class="o">=</span><span class="s2">"children"</span><span class="p">)</span>
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<span class="n">born_at</span><span class="p">:</span> <span class="n">Mapped</span><span class="p">[</span><span class="n">datetime</span><span class="p">]</span> <span class="o">=</span> <span class="n">mapped_column</span><span class="p">()</span>
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</code></pre></div>
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<p>I’ve tried many ways, with complex subqueries and the like, before finding out <a href="https://www.sqlitetutorial.net/sqlite-distinct/">the <span class="caps">DISTINCT</span> <span class="caps">SQL</span> statement</a>.</p>
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<p>So, if you want to retrieve the parent with it’s more recent child, you can do it like this:</p>
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<div class="highlight"><pre><span></span><code><span class="n">results</span> <span class="o">=</span> <span class="p">(</span>
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<span class="n">db</span><span class="o">.</span><span class="n">query</span><span class="p">(</span><span class="n">Parent</span><span class="p">,</span> <span class="n">Child</span><span class="p">)</span>
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<span class="o">.</span><span class="n">join</span><span class="p">(</span><span class="n">Child</span><span class="p">)</span>
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<span class="o">.</span><span class="n">distinct</span><span class="p">(</span><span class="n">Parent</span><span class="o">.</span><span class="n">id</span><span class="p">)</span>
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<span class="o">.</span><span class="n">order_by</span><span class="p">(</span><span class="n">Parent</span><span class="o">.</span><span class="n">id</span><span class="p">,</span> <span class="n">desc</span><span class="p">(</span><span class="n">Child</span><span class="o">.</span><span class="n">born_at</span><span class="p">))</span>
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<span class="o">.</span><span class="n">all</span><span class="p">()</span>
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<span class="p">)</span>
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</code></pre></div>
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<p>
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<a href="https://blog.notmyidea.org/tag/sql.html">#SQL</a>, <a href="https://blog.notmyidea.org/tag/sqlalchemy.html">#SQLAlchemy</a>, <a href="https://blog.notmyidea.org/tag/python.html">#Python</a> - Posté dans la catégorie <a href="https://blog.notmyidea.org/code/">code</a>
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