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<div class="yui-g" id="howto-writing-migrations">
<div class="section" id="s-writing-database-migrations">
<span id="writing-database-migrations"></span><h1>Writing database migrations<a class="headerlink" href="#writing-database-migrations" title="Permalink to this headline">¶</a></h1>
<p>This document explains how to structure and write database migrations for
different scenarios you might encounter. For introductory material on
migrations, see <a class="reference internal" href="../topics/migrations.html"><span class="doc">the topic guide</span></a>.</p>
<div class="section" id="s-data-migrations-and-multiple-databases">
<span id="s-id1"></span><span id="data-migrations-and-multiple-databases"></span><span id="id1"></span><h2>Data migrations and multiple databases<a class="headerlink" href="#data-migrations-and-multiple-databases" title="Permalink to this headline">¶</a></h2>
<p>When using multiple databases, you may need to figure out whether or not to
run a migration against a particular database. For example, you may want to
<strong>only</strong> run a migration on a particular database.</p>
<p>In order to do that you can check the database connection’s alias inside a
<code class="docutils literal"><span class="pre">RunPython</span></code> operation by looking at the <code class="docutils literal"><span class="pre">schema_editor.connection.alias</span></code>
attribute:</p>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">django.db</span> <span class="k">import</span> <span class="n">migrations</span>
<span class="k">def</span> <span class="nf">forwards</span><span class="p">(</span><span class="n">apps</span><span class="p">,</span> <span class="n">schema_editor</span><span class="p">):</span>
<span class="k">if</span> <span class="ow">not</span> <span class="n">schema_editor</span><span class="o">.</span><span class="n">connection</span><span class="o">.</span><span class="n">alias</span> <span class="o">==</span> <span class="s1">'default'</span><span class="p">:</span>
<span class="k">return</span>
<span class="c1"># Your migration code goes here</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="c1"># Dependencies to other migrations</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">RunPython</span><span class="p">(</span><span class="n">forwards</span><span class="p">),</span>
<span class="p">]</span>
</pre></div>
</div>
<div class="versionadded">
<span class="title">New in Django 1.8.</span> </div>
<p>You can also provide hints that will be passed to the <a class="reference internal" href="../topics/db/multi-db.html#allow_migrate" title="allow_migrate"><code class="xref py py-meth docutils literal"><span class="pre">allow_migrate()</span></code></a>
method of database routers as <code class="docutils literal"><span class="pre">**hints</span></code>:</p>
<div class="highlight-default"><div class="snippet-filename">myapp/dbrouters.py</div>
<div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">MyRouter</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span>
<span class="k">def</span> <span class="nf">allow_migrate</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">db</span><span class="p">,</span> <span class="n">app_label</span><span class="p">,</span> <span class="n">model_name</span><span class="o">=</span><span class="kc">None</span><span class="p">,</span> <span class="o">**</span><span class="n">hints</span><span class="p">):</span>
<span class="k">if</span> <span class="s1">'target_db'</span> <span class="ow">in</span> <span class="n">hints</span><span class="p">:</span>
<span class="k">return</span> <span class="n">db</span> <span class="o">==</span> <span class="n">hints</span><span class="p">[</span><span class="s1">'target_db'</span><span class="p">]</span>
<span class="k">return</span> <span class="kc">True</span>
</pre></div>
</div>
<p>Then, to leverage this in your migrations, do the following:</p>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">django.db</span> <span class="k">import</span> <span class="n">migrations</span>
<span class="k">def</span> <span class="nf">forwards</span><span class="p">(</span><span class="n">apps</span><span class="p">,</span> <span class="n">schema_editor</span><span class="p">):</span>
<span class="c1"># Your migration code goes here</span>
<span class="o">...</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="c1"># Dependencies to other migrations</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">RunPython</span><span class="p">(</span><span class="n">forwards</span><span class="p">,</span> <span class="n">hints</span><span class="o">=</span><span class="p">{</span><span class="s1">'target_db'</span><span class="p">:</span> <span class="s1">'default'</span><span class="p">}),</span>
<span class="p">]</span>
</pre></div>
</div>
<p>If your <code class="docutils literal"><span class="pre">RunPython</span></code> or <code class="docutils literal"><span class="pre">RunSQL</span></code> operation only affects one model, it’s good
practice to pass <code class="docutils literal"><span class="pre">model_name</span></code> as a hint to make it as transparent as possible
to the router. This is especially important for reusable and third-party apps.</p>
</div>
<div class="section" id="s-migrations-that-add-unique-fields">
<span id="migrations-that-add-unique-fields"></span><h2>Migrations that add unique fields<a class="headerlink" href="#migrations-that-add-unique-fields" title="Permalink to this headline">¶</a></h2>
<p>Applying a “plain” migration that adds a unique non-nullable field to a table
with existing rows will raise an error because the value used to populate
existing rows is generated only once, thus breaking the unique constraint.</p>
<p>Therefore, the following steps should be taken. In this example, we’ll add a
non-nullable <a class="reference internal" href="../ref/models/fields.html#django.db.models.UUIDField" title="django.db.models.UUIDField"><code class="xref py py-class docutils literal"><span class="pre">UUIDField</span></code></a> with a default value. Modify
the respective field according to your needs.</p>
<ul>
<li><p class="first">Add the field on your model with <code class="docutils literal"><span class="pre">default=uuid.uuid4</span></code> and <code class="docutils literal"><span class="pre">unique=True</span></code>
arguments (choose an appropriate default for the type of the field you’re
adding).</p>
</li>
<li><p class="first">Run the <a class="reference internal" href="../ref/django-admin.html#django-admin-makemigrations"><code class="xref std std-djadmin docutils literal"><span class="pre">makemigrations</span></code></a> command. This should generate a migration
with an <code class="docutils literal"><span class="pre">AddField</span></code> operation.</p>
</li>
<li><p class="first">Generate two empty migration files for the same app by running
<code class="docutils literal"><span class="pre">makemigrations</span> <span class="pre">myapp</span> <span class="pre">--empty</span></code> twice. We’ve renamed the migration files to
give them meaningful names in the examples below.</p>
</li>
<li><p class="first">Copy the <code class="docutils literal"><span class="pre">AddField</span></code> operation from the auto-generated migration (the first
of the three new files) to the last migration and change <code class="docutils literal"><span class="pre">AddField</span></code> to
<code class="docutils literal"><span class="pre">AlterField</span></code>. For example:</p>
<div class="highlight-default"><div class="snippet-filename">0006_remove_uuid_null.py</div>
<div class="highlight"><pre><span></span><span class="c1"># -*- coding: utf-8 -*-</span>
<span class="kn">from</span> <span class="nn">__future__</span> <span class="k">import</span> <span class="n">unicode_literals</span>
<span class="kn">from</span> <span class="nn">django.db</span> <span class="k">import</span> <span class="n">migrations</span><span class="p">,</span> <span class="n">models</span>
<span class="kn">import</span> <span class="nn">uuid</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">'myapp'</span><span class="p">,</span> <span class="s1">'0005_populate_uuid_values'</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">AlterField</span><span class="p">(</span>
<span class="n">model_name</span><span class="o">=</span><span class="s1">'mymodel'</span><span class="p">,</span>
<span class="n">name</span><span class="o">=</span><span class="s1">'uuid'</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">UUIDField</span><span class="p">(</span><span class="n">default</span><span class="o">=</span><span class="n">uuid</span><span class="o">.</span><span class="n">uuid4</span><span class="p">,</span> <span class="n">unique</span><span class="o">=</span><span class="kc">True</span><span class="p">),</span>
<span class="p">),</span>
<span class="p">]</span>
</pre></div>
</div>
</li>
<li><p class="first">Edit the first migration file. The generated migration class should look
similar to this:</p>
<div class="highlight-default"><div class="snippet-filename">0004_add_uuid_field.py</div>
<div class="highlight"><pre><span></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">'myapp'</span><span class="p">,</span> <span class="s1">'0003_auto_20150129_1705'</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">AddField</span><span class="p">(</span>
<span class="n">model_name</span><span class="o">=</span><span class="s1">'mymodel'</span><span class="p">,</span>
<span class="n">name</span><span class="o">=</span><span class="s1">'uuid'</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">UUIDField</span><span class="p">(</span><span class="n">default</span><span class="o">=</span><span class="n">uuid</span><span class="o">.</span><span class="n">uuid4</span><span class="p">,</span> <span class="n">unique</span><span class="o">=</span><span class="kc">True</span><span class="p">),</span>
<span class="p">),</span>
<span class="p">]</span>
</pre></div>
</div>
<p>Change <code class="docutils literal"><span class="pre">unique=True</span></code> to <code class="docutils literal"><span class="pre">null=True</span></code> – this will create the intermediary
null field and defer creating the unique constraint until we’ve populated
unique values on all the rows.</p>
</li>
<li><p class="first">In the first empty migration file, add a
<a class="reference internal" href="../ref/migration-operations.html#django.db.migrations.operations.RunPython" title="django.db.migrations.operations.RunPython"><code class="xref py py-class docutils literal"><span class="pre">RunPython</span></code></a> or
<a class="reference internal" href="../ref/migration-operations.html#django.db.migrations.operations.RunSQL" title="django.db.migrations.operations.RunSQL"><code class="xref py py-class docutils literal"><span class="pre">RunSQL</span></code></a> operation to generate a
unique value (UUID in the example) for each existing row. For example:</p>
<div class="highlight-default"><div class="snippet-filename">0005_populate_uuid_values.py</div>
<div class="highlight"><pre><span></span><span class="c1"># -*- coding: utf-8 -*-</span>
<span class="kn">from</span> <span class="nn">__future__</span> <span class="k">import</span> <span class="n">unicode_literals</span>
<span class="kn">from</span> <span class="nn">django.db</span> <span class="k">import</span> <span class="n">migrations</span><span class="p">,</span> <span class="n">models</span>
<span class="kn">import</span> <span class="nn">uuid</span>
<span class="k">def</span> <span class="nf">gen_uuid</span><span class="p">(</span><span class="n">apps</span><span class="p">,</span> <span class="n">schema_editor</span><span class="p">):</span>
<span class="n">MyModel</span> <span class="o">=</span> <span class="n">apps</span><span class="o">.</span><span class="n">get_model</span><span class="p">(</span><span class="s1">'myapp'</span><span class="p">,</span> <span class="s1">'MyModel'</span><span class="p">)</span>
<span class="k">for</span> <span class="n">row</span> <span class="ow">in</span> <span class="n">MyModel</span><span class="o">.</span><span class="n">objects</span><span class="o">.</span><span class="n">all</span><span class="p">():</span>
<span class="n">row</span><span class="o">.</span><span class="n">uuid</span> <span class="o">=</span> <span class="n">uuid</span><span class="o">.</span><span class="n">uuid4</span><span class="p">()</span>
<span class="n">row</span><span class="o">.</span><span class="n">save</span><span class="p">()</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">'myapp'</span><span class="p">,</span> <span class="s1">'0004_add_uuid_field'</span><span class="p">),</span>
<span class="p">]</span>
<span class="n">operations</span> <span class="o">=</span> <span class="p">[</span>
<span class="c1"># omit reverse_code=... if you don't want the migration to be reversible.</span>
<span class="n">migrations</span><span class="o">.</span><span class="n">RunPython</span><span class="p">(</span><span class="n">gen_uuid</span><span class="p">,</span> <span class="n">reverse_code</span><span class="o">=</span><span class="n">migrations</span><span class="o">.</span><span class="n">RunPython</span><span class="o">.</span><span class="n">noop</span><span class="p">),</span>
<span class="p">]</span>
</pre></div>
</div>
</li>
<li><p class="first">Now you can apply the migrations as usual with the <a class="reference internal" href="../ref/django-admin.html#django-admin-migrate"><code class="xref std std-djadmin docutils literal"><span class="pre">migrate</span></code></a> command.</p>
<p>Note there is a race condition if you allow objects to be created while this
migration is running. Objects created after the <code class="docutils literal"><span class="pre">AddField</span></code> and before
<code class="docutils literal"><span class="pre">RunPython</span></code> will have their original <code class="docutils literal"><span class="pre">uuid</span></code>’s overwritten.</p>
</li>
</ul>
</div>
<div class="section" id="s-controlling-the-order-of-migrations">
<span id="controlling-the-order-of-migrations"></span><h2>Controlling the order of migrations<a class="headerlink" href="#controlling-the-order-of-migrations" title="Permalink to this headline">¶</a></h2>
<p>Django determines the order in which migrations should be applied not by the
filename of each migration, but by building a graph using two properties on the
<code class="docutils literal"><span class="pre">Migration</span></code> class: <code class="docutils literal"><span class="pre">dependencies</span></code> and <code class="docutils literal"><span class="pre">run_before</span></code>.</p>
<p>If you’ve used the <a class="reference internal" href="../ref/django-admin.html#django-admin-makemigrations"><code class="xref std std-djadmin docutils literal"><span class="pre">makemigrations</span></code></a> command you’ve probably
already seen <code class="docutils literal"><span class="pre">dependencies</span></code> in action because auto-created
migrations have this defined as part of their creation process.</p>
<p>The <code class="docutils literal"><span class="pre">dependencies</span></code> property is declared like this:</p>
<div class="highlight-default"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">django.db</span> <span class="k">import</span> <span class="n">migrations</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">'myapp'</span><span class="p">,</span> <span class="s1">'0123_the_previous_migration'</span><span class="p">),</span>
<span class="p">]</span>
</pre></div>
</div>
<p>Usually this will be enough, but from time to time you may need to
ensure that your migration runs <em>before</em> other migrations. This is
useful, for example, to make third-party apps’ migrations run <em>after</em>
your <a class="reference internal" href="../ref/settings.html#std:setting-AUTH_USER_MODEL"><code class="xref std std-setting docutils literal"><span class="pre">AUTH_USER_MODEL</span></code></a> replacement.</p>
<p>To achieve this, place all migrations that should depend on yours in
the <code class="docutils literal"><span class="pre">run_before</span></code> attribute on your <code class="docutils literal"><span class="pre">Migration</span></code> class:</p>
<div class="highlight-default"><div class="highlight"><pre><span></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="o">...</span>
<span class="n">run_before</span> <span class="o">=</span> <span class="p">[</span>
<span class="p">(</span><span class="s1">'third_party_app'</span><span class="p">,</span> <span class="s1">'0001_do_awesome'</span><span class="p">),</span>
<span class="p">]</span>
</pre></div>
</div>
<p>Prefer using <code class="docutils literal"><span class="pre">dependencies</span></code> over <code class="docutils literal"><span class="pre">run_before</span></code> when possible. You should
only use <code class="docutils literal"><span class="pre">run_before</span></code> if it is undesirable or impractical to specify
<code class="docutils literal"><span class="pre">dependencies</span></code> in the migration which you want to run after the one you are
writing.</p>
</div>
</div>
</div>
</div>
</div>
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<h3><a href="../contents.html">Table Of Contents</a></h3>
<ul>
<li><a class="reference internal" href="#">Writing database migrations</a><ul>
<li><a class="reference internal" href="#data-migrations-and-multiple-databases">Data migrations and multiple databases</a></li>
<li><a class="reference internal" href="#migrations-that-add-unique-fields">Migrations that add unique fields</a></li>
<li><a class="reference internal" href="#controlling-the-order-of-migrations">Controlling the order of migrations</a></li>
</ul>
</li>
</ul>
<h3>Browse</h3>
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