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<title>Django model: change field name with no DB impact - Thomas Loiret - Random thoughts</title>
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<h1 itemprop="name">Django model: change field name with no DB impact</h1>
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Thomas Loiret
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<span class="meta__date" itemprop="datePublished" content="2021-03-06">
<span class="glyphicon glyphicon-calendar"></span> Sat 06 March 2021
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<p><strong>TL;DR:</strong> Use <a href="https://docs.djangoproject.com/en/dev/ref/models/fields/#db-column"><code>db_column</code></a> and <a href="https://docs.djangoproject.com/en/dev/ref/migration-operations/#django.db.migrations.operations.SeparateDatabaseAndState"><code>SeparateDatabaseAndState</code></a>.</p>
<h2>The issue we are trying to solve</h2>
<p>Let say we have a model</p>
<div class="highlight"><pre><span></span><code><span class="k">class</span> <span class="nc">Recipe</span><span class="p">(</span><span class="n">models</span><span class="o">.</span><span class="n">Model</span><span class="p">):</span>
<span class="n">title</span> <span class="o">=</span> <span class="n">models</span><span class="o">.</span><span class="n">CharField</span><span class="p">(</span><span class="n">max_length</span><span class="o">=</span><span class="mi">30</span><span class="p">)</span>
</code></pre></div>
<p>and we want to change the field from <code>title</code> to <code>name</code>:</p>
<div class="highlight"><pre><span></span><code><span class="k">class</span> <span class="nc">Recipe</span><span class="p">(</span><span class="n">models</span><span class="o">.</span><span class="n">Model</span><span class="p">):</span>
<span class="n">name</span> <span class="o">=</span> <span class="n">models</span><span class="o">.</span><span class="n">CharField</span><span class="p">(</span><span class="n">max_length</span><span class="o">=</span><span class="mi">30</span><span class="p">)</span>
</code></pre></div>
<p>Django will generate a migration to handle everything nicely. It works, but we might have some issues between the code deployment and the migration execution. During this time, the Django application will generate an error everytime this model is used because of the inconsistent state between the code and the database.</p>
<p>In this article, we will try to avoid any downtime when doing the change.</p>
<h2>Use db_column</h2>
<div class="highlight"><pre><span></span><code><span class="k">class</span> <span class="nc">Recipe</span><span class="p">(</span><span class="n">models</span><span class="o">.</span><span class="n">Model</span><span class="p">):</span>
<span class="n">name</span> <span class="o">=</span> <span class="n">models</span><span class="o">.</span><span class="n">CharField</span><span class="p">(</span><span class="n">max_length</span><span class="o">=</span><span class="mi">30</span><span class="p">,</span> <span class="n">db_colum</span><span class="o">=</span><span class="s2">"title"</span><span class="p">)</span>
</code></pre></div>
<p>The attribute <a href="https://docs.djangoproject.com/en/dev/ref/models/fields/#db-column"><code>db_column</code></a> indicates the name of the column in database (obviously). So, by doing this we won't affect it.</p>
<p>Django recognizes the renaming and generates the migration.</p>
<div class="highlight"><pre><span></span><code>./manage.py makemigrations
Did you rename recipe.title to recipe.name (a CharField)? [y/N]
Migrations for 'recipes':
recipes/migrations/0004_auto_20210305_2021.py
<span class="w"> </span><span class="k">-</span><span class="w"> </span>Change Meta options on recipe
<span class="w"> </span><span class="k">-</span><span class="w"> </span>Rename field title on recipe to name
<span class="w"> </span><span class="k">-</span><span class="w"> </span>Alter field name on title
</code></pre></div>
<p>But if we look at the SQL generated by this migration, it may seem a little odd:</p>
<div class="highlight"><pre><span></span><code>./manage.py sqlmigrate recipes 0004_auto_20210305_2021
<span class="gu">BEGIN;</span>
<span class="gu">--</span>
<span class="gu">-- Rename field title on recipe to name</span>
<span class="gu">--</span>
<span class="gu">ALTER TABLE "recipes_recipe" RENAME COLUMN "title" TO "name";</span>
<span class="gu">--</span>
<span class="gu">-- Alter field name on recipe</span>
<span class="gu">--</span>
ALTER TABLE "recipes_recipe" RENAME COLUMN "name" TO "title";
COMMIT;
</code></pre></div>
<p>The column is renamed twice to keep its original name. Why not doing nothing instead? After a bit of searching, it seems like this issue has already been raised in this Django <a href="https://code.djangoproject.com/ticket/24260">ticket</a> but no solution was found.</p>
<p>Anyway, even if this migration is a bit surprising, it does resolve our issue. There will be no downtime!</p>
<h2>Even better, use SeparateDatabaseAndState</h2>
<p>As a bonus, we will try to avoid any SQL operations since the rename should not affect the database. If we take a look at the migration generated, it should look like:</p>
<div class="highlight"><pre><span></span><code><span class="kn">from</span> <span class="nn">django.db</span> <span class="kn">import</span> <span class="n">migrations</span><span class="p">,</span> <span class="n">models</span>
<span class="k">class</span> <span class="nc">Migration</span><span class="p">(</span><span class="n">migrations</span><span class="o">.</span><span class="n">Migration</span><span class="p">):</span>
<span class="n">dependencies</span> <span class="o">=</span> <span class="p">[</span>
<span class="p">(</span><span class="s1">'recipes'</span><span class="p">,</span> <span class="s1">'0003_auto_20210226_1437'</span><span class="p">),</span>
<span class="p">]</span>
<span class="n">operations</span> <span class="o">=</span> <span class="p">[</span>
<span class="n">migrations</span><span class="o">.</span><span class="n">RenameField</span><span class="p">(</span>
<span class="n">model_name</span><span class="o">=</span><span class="s1">'recipe'</span><span class="p">,</span>
<span class="n">old_name</span><span class="o">=</span><span class="s1">'title'</span><span class="p">,</span>
<span class="n">new_name</span><span class="o">=</span><span class="s1">'name'</span><span class="p">,</span>
<span class="p">),</span>
<span class="n">migrations</span><span class="o">.</span><span class="n">AlterField</span><span class="p">(</span>
<span class="n">model_name</span><span class="o">=</span><span class="s1">'recipe'</span><span class="p">,</span>
<span class="n">name</span><span class="o">=</span><span class="s1">'name'</span><span class="p">,</span>
<span class="n">field</span><span class="o">=</span><span class="n">models</span><span class="o">.</span><span class="n">CharField</span><span class="p">(</span><span class="n">db_column</span><span class="o">=</span><span class="s1">'title'</span><span class="p">,</span> <span class="n">max_length</span><span class="o">=</span><span class="mi">30</span><span class="p">),</span>
<span class="p">),</span>
<span class="p">]</span>
</code></pre></div>
<p>We now understand why there is a "double renaming" in the SQL generated. Django considers that 2 operations happened. The renaming first and then the addition of the <a href="https://docs.djangoproject.com/en/dev/ref/models/fields/#db-column"><code>db_column</code></a> attribute.</p>
<p>We know that these 2 operations should have no effect on the database and we can inform Django by using <a href="https://docs.djangoproject.com/en/dev/ref/migration-operations/#django.db.migrations.operations.SeparateDatabaseAndState"><code>SeparateDatabaseAndState</code></a>. We keep <code>database_operations</code> empty to indicate that nothing should be done in database and we put the operations generated by Django in <code>state_operations</code>. That way everyone is happy!</p>
<div class="highlight"><pre><span></span><code><span class="k">class</span> <span class="nc">Migration</span><span class="p">(</span><span class="n">migrations</span><span class="o">.</span><span class="n">Migration</span><span class="p">):</span>
<span class="n">dependencies</span> <span class="o">=</span> <span class="p">[</span>
<span class="p">(</span><span class="s2">"recipes"</span><span class="p">,</span> <span class="s2">"0003_auto_20210226_1437"</span><span class="p">),</span>
<span class="p">]</span>
<span class="n">operations</span> <span class="o">=</span> <span class="p">[</span>
<span class="n">migrations</span><span class="o">.</span><span class="n">SeparateDatabaseAndState</span><span class="p">(</span>
<span class="n">database_operations</span><span class="o">=</span><span class="p">[],</span>
<span class="n">state_operations</span><span class="o">=</span><span class="p">[</span>
<span class="n">migrations</span><span class="o">.</span><span class="n">RenameField</span><span class="p">(</span>
<span class="n">model_name</span><span class="o">=</span><span class="s1">'recipe'</span><span class="p">,</span>
<span class="n">old_name</span><span class="o">=</span><span class="s1">'title'</span><span class="p">,</span>
<span class="n">new_name</span><span class="o">=</span><span class="s1">'name'</span><span class="p">,</span>
<span class="p">),</span>
<span class="n">migrations</span><span class="o">.</span><span class="n">AlterField</span><span class="p">(</span>
<span class="n">model_name</span><span class="o">=</span><span class="s1">'recipe'</span><span class="p">,</span>
<span class="n">name</span><span class="o">=</span><span class="s1">'name'</span><span class="p">,</span>
<span class="n">field</span><span class="o">=</span><span class="n">models</span><span class="o">.</span><span class="n">CharField</span><span class="p">(</span><span class="n">db_column</span><span class="o">=</span><span class="s1">'title'</span><span class="p">,</span> <span class="n">max_length</span><span class="o">=</span><span class="mi">30</span><span class="p">),</span>
<span class="p">),</span>
<span class="p">],</span>
<span class="p">)</span>
<span class="p">]</span>
</code></pre></div>
<p>And we can check that nothing is done:</p>
<div class="highlight"><pre><span></span><code>./manage.py sqlmigrate recipes 0004_auto_20210305_2021
<span class="gu">BEGIN;</span>
<span class="gu">--</span>
<span class="gu">-- Custom state/database change combination</span>
<span class="gu">--</span>
COMMIT;
</code></pre></div>
<p>That's all :)</p>
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