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<title>Django: avoid using .first() when retrieving a unique object - Thomas Loiret - Random thoughts</title>
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<h1 itemprop="name">Django: avoid using .first() when retrieving a unique object</h1>
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Thomas Loiret
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<span class="meta__date" itemprop="datePublished" content="2024-12-18">
<span class="glyphicon glyphicon-calendar"></span> Wed 18 December 2024
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<p>When we want to retrieve an object from DB we do:</p>
<div class="highlight"><pre><span></span><code><span class="n">obj</span> <span class="o">=</span> <span class="n">MyModel</span><span class="o">.</span><span class="n">objects</span><span class="o">.</span><span class="n">get</span><span class="p">(</span><span class="n">unique_identifier</span><span class="o">=</span><span class="mi">999</span><span class="p">)</span>
</code></pre></div>
<p>But if the object may or may not exist, then we might do:</p>
<div class="highlight"><pre><span></span><code><span class="k">try</span><span class="p">:</span>
<span class="n">obj</span> <span class="o">=</span> <span class="n">MyModel</span><span class="o">.</span><span class="n">objects</span><span class="o">.</span><span class="n">get</span><span class="p">(</span><span class="n">unique_identifier</span><span class="o">=</span><span class="mi">999</span><span class="p">)</span>
<span class="k">except</span> <span class="n">MyModel</span><span class="o">.</span><span class="n">DoesNotExist</span><span class="p">:</span>
<span class="n">obj</span> <span class="o">=</span> <span class="kc">None</span>
</code></pre></div>
<p>At this point, repeating this pattern of try/except in a lot of places might seems a bit of a burden. Then, we have this nice way of doing it:</p>
<div class="highlight"><pre><span></span><code><span class="n">obj</span> <span class="o">=</span> <span class="n">MyModel</span><span class="o">.</span><span class="n">objects</span><span class="o">.</span><span class="n">filter</span><span class="p">(</span><span class="n">unique_identifier</span><span class="o">=</span><span class="mi">999</span><span class="p">)</span><span class="o">.</span><span class="n">first</span><span class="p">()</span>
</code></pre></div>
<p>It is quite handy and pythonic because here <code>obj</code> will either contain the object we want or be <code>None</code> if we did not find it. Only one line and no more <code>try</code>/<code>except</code>!</p>
<p>The downside here are the performances. When we use <code>.first()</code> (or <code>.last()</code>) Django automatically adds an ordering to get the first matching object given that ordering. That seems logical, but on large table, ordering is expensive and for this specific case where we know that we will retrieve at most 1 object it is useless.</p>
<h2>Show me the SQL</h2>
<p>For example, using <code>.first()</code> gives us:</p>
<div class="highlight"><pre><span></span><code><span class="c1"># Adding .only('id') to keep the SQL short</span>
<span class="n">user</span> <span class="o">=</span> <span class="n">User</span><span class="o">.</span><span class="n">objects</span><span class="o">.</span><span class="n">only</span><span class="p">(</span><span class="s1">'id'</span><span class="p">)</span><span class="o">.</span><span class="n">filter</span><span class="p">(</span><span class="n">username</span><span class="o">=</span><span class="s1">'foo'</span><span class="p">)</span><span class="o">.</span><span class="n">first</span><span class="p">()</span>
</code></pre></div>
<div class="highlight"><pre><span></span><code><span class="k">SELECT</span><span class="w"> </span><span class="ss">"auth_user"</span><span class="p">.</span><span class="ss">"id"</span>
<span class="k">FROM</span><span class="w"> </span><span class="ss">"auth_user"</span>
<span class="k">WHERE</span><span class="w"> </span><span class="ss">"auth_user"</span><span class="p">.</span><span class="ss">"username"</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="s1">'foo'</span>
<span class="k">ORDER</span><span class="w"> </span><span class="k">BY</span><span class="w"> </span><span class="ss">"auth_user"</span><span class="p">.</span><span class="ss">"id"</span><span class="w"> </span><span class="k">ASC</span>
<span class="k">LIMIT</span><span class="w"> </span><span class="mi">1</span>
</code></pre></div>
<p>When the SQL generated by <code>.get()</code> gives us:</p>
<div class="highlight"><pre><span></span><code><span class="k">try</span><span class="p">:</span>
<span class="c1"># Adding .only('id') to keep the SQL short</span>
<span class="n">user</span> <span class="o">=</span> <span class="n">User</span><span class="o">.</span><span class="n">objects</span><span class="o">.</span><span class="n">only</span><span class="p">(</span><span class="s1">'id'</span><span class="p">)</span><span class="o">.</span><span class="n">get</span><span class="p">(</span><span class="n">username</span><span class="o">=</span><span class="s1">'foo'</span><span class="p">)</span>
<span class="k">except</span> <span class="n">User</span><span class="o">.</span><span class="n">DoesNotExist</span><span class="p">:</span>
<span class="n">user</span> <span class="o">=</span> <span class="kc">None</span>
</code></pre></div>
<div class="highlight"><pre><span></span><code><span class="k">SELECT</span><span class="w"> </span><span class="ss">"auth_user"</span><span class="p">.</span><span class="ss">"id"</span>
<span class="k">FROM</span><span class="w"> </span><span class="ss">"auth_user"</span>
<span class="k">WHERE</span><span class="w"> </span><span class="ss">"auth_user"</span><span class="p">.</span><span class="ss">"username"</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="s1">'foo'</span>
<span class="k">LIMIT</span><span class="w"> </span><span class="mi">21</span>
</code></pre></div>
<p>No more ordering, it is faster!</p>
<h2>Extra</h2>
<p>Noticed in the last example that the limit was 21 when it could be 1? The following code will generate a more accurate SQL:</p>
<div class="highlight"><pre><span></span><code><span class="n">user_qs</span> <span class="o">=</span> <span class="n">User</span><span class="o">.</span><span class="n">objects</span><span class="o">.</span><span class="n">only</span><span class="p">(</span><span class="s1">'id'</span><span class="p">)</span><span class="o">.</span><span class="n">filter</span><span class="p">(</span><span class="n">username</span><span class="o">=</span><span class="s1">'foo'</span><span class="p">)[:</span><span class="mi">1</span><span class="p">]</span>
<span class="n">user</span> <span class="o">=</span> <span class="n">user_qs</span><span class="p">[</span><span class="mi">0</span><span class="p">]</span> <span class="k">if</span> <span class="n">user_qs</span> <span class="k">else</span> <span class="kc">None</span>
</code></pre></div>
<div class="highlight"><pre><span></span><code><span class="k">SELECT</span><span class="w"> </span><span class="ss">"auth_user"</span><span class="p">.</span><span class="ss">"id"</span>
<span class="k">FROM</span><span class="w"> </span><span class="ss">"auth_user"</span>
<span class="k">WHERE</span><span class="w"> </span><span class="ss">"auth_user"</span><span class="p">.</span><span class="ss">"username"</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="s1">'foo'</span>
<span class="k">LIMIT</span><span class="w"> </span><span class="mi">1</span>
</code></pre></div>
<p>But the python code is hardly readable, I would not recommend it unless you know what you are doing.</p>
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