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  <div class="section" id="database-schema-migrations">
<h1>Database schema migrations<a class="headerlink" href="#database-schema-migrations" title="Permalink to this headline"></a></h1>
<p>Importing <a class="reference internal" href="api.html#module-migrate.changeset" title="migrate.changeset: Database changeset management"><tt class="xref py py-mod docutils literal"><span class="pre">migrate.changeset</span></tt></a> adds some new methods to existing SQLAlchemy
objects, as well as creating functions of its own. Most operations can be done
either by a method or a function. Methods match SQLAlchemy&#8217;s existing API and
are more intuitive when the object is available; functions allow one to make
changes when only the name of an object is available (for example, adding a
column to a table in the database without having to load that table into
Python).</p>
<p>Changeset operations can be used independently of SQLAlchemy Migrate&#8217;s
<a class="reference internal" href="versioning.html#versioning-system"><em>versioning</em></a>.</p>
<p>For more information, see the API documentation for <a class="reference internal" href="api.html#module-migrate.changeset" title="migrate.changeset: Database changeset management"><tt class="xref py py-mod docutils literal"><span class="pre">migrate.changeset</span></tt></a>.</p>
<p id="summary-changeset-api">Here are some direct links to the relevent sections of the API documentations:</p>
<ul class="simple">
<li><a class="reference internal" href="api.html#migrate.changeset.schema.ChangesetColumn.create" title="migrate.changeset.schema.ChangesetColumn.create"><tt class="xref py py-meth docutils literal"><span class="pre">Create</span> <span class="pre">a</span> <span class="pre">column</span></tt></a></li>
<li><a class="reference internal" href="api.html#migrate.changeset.schema.ChangesetColumn.drop" title="migrate.changeset.schema.ChangesetColumn.drop"><tt class="xref py py-meth docutils literal"><span class="pre">Drop</span> <span class="pre">a</span> <span class="pre">column</span></tt></a></li>
<li><a class="reference internal" href="api.html#migrate.changeset.schema.ChangesetColumn.alter" title="migrate.changeset.schema.ChangesetColumn.alter"><tt class="xref py py-meth docutils literal"><span class="pre">Alter</span> <span class="pre">a</span> <span class="pre">column</span></tt></a> (follow a link for list of supported changes)</li>
<li><a class="reference internal" href="api.html#migrate.changeset.schema.ChangesetTable.rename" title="migrate.changeset.schema.ChangesetTable.rename"><tt class="xref py py-meth docutils literal"><span class="pre">Rename</span> <span class="pre">a</span> <span class="pre">table</span></tt></a></li>
<li><a class="reference internal" href="api.html#migrate.changeset.schema.ChangesetIndex.rename" title="migrate.changeset.schema.ChangesetIndex.rename"><tt class="xref py py-meth docutils literal"><span class="pre">Rename</span> <span class="pre">an</span> <span class="pre">index</span></tt></a></li>
<li><a class="reference internal" href="api.html#migrate.changeset.constraint.PrimaryKeyConstraint" title="migrate.changeset.constraint.PrimaryKeyConstraint"><tt class="xref py py-meth docutils literal"><span class="pre">Create</span> <span class="pre">primary</span> <span class="pre">key</span> <span class="pre">constraint</span></tt></a></li>
<li><a class="reference internal" href="api.html#migrate.changeset.constraint.PrimaryKeyConstraint.drop" title="migrate.changeset.constraint.PrimaryKeyConstraint.drop"><tt class="xref py py-meth docutils literal"><span class="pre">Drop</span> <span class="pre">primary</span> <span class="pre">key</span> <span class="pre">constraint</span></tt></a></li>
<li><a class="reference internal" href="api.html#migrate.changeset.constraint.ForeignKeyConstraint.create" title="migrate.changeset.constraint.ForeignKeyConstraint.create"><tt class="xref py py-meth docutils literal"><span class="pre">Create</span> <span class="pre">foreign</span> <span class="pre">key</span> <span class="pre">contraint</span></tt></a></li>
<li><a class="reference internal" href="api.html#migrate.changeset.constraint.ForeignKeyConstraint.drop" title="migrate.changeset.constraint.ForeignKeyConstraint.drop"><tt class="xref py py-meth docutils literal"><span class="pre">Drop</span> <span class="pre">foreign</span> <span class="pre">key</span> <span class="pre">constraint</span></tt></a></li>
<li><a class="reference internal" href="api.html#migrate.changeset.constraint.UniqueConstraint.create" title="migrate.changeset.constraint.UniqueConstraint.create"><tt class="xref py py-meth docutils literal"><span class="pre">Create</span> <span class="pre">unique</span> <span class="pre">key</span> <span class="pre">contraint</span></tt></a></li>
<li><a class="reference internal" href="api.html#migrate.changeset.constraint.UniqueConstraint.drop" title="migrate.changeset.constraint.UniqueConstraint.drop"><tt class="xref py py-meth docutils literal"><span class="pre">Drop</span> <span class="pre">unique</span> <span class="pre">key</span> <span class="pre">constraint</span></tt></a></li>
<li><a class="reference internal" href="api.html#migrate.changeset.constraint.CheckConstraint.create" title="migrate.changeset.constraint.CheckConstraint.create"><tt class="xref py py-meth docutils literal"><span class="pre">Create</span> <span class="pre">check</span> <span class="pre">key</span> <span class="pre">contraint</span></tt></a></li>
<li><a class="reference internal" href="api.html#migrate.changeset.constraint.CheckConstraint.drop" title="migrate.changeset.constraint.CheckConstraint.drop"><tt class="xref py py-meth docutils literal"><span class="pre">Drop</span> <span class="pre">check</span> <span class="pre">key</span> <span class="pre">constraint</span></tt></a></li>
</ul>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p class="last">Many of the schema modification methods above take an <tt class="docutils literal"><span class="pre">alter_metadata</span></tt>
keyword parameter. This parameter defaults to <cite>True</cite>.</p>
</div>
<p>The following sections give examples of how to make various kinds of schema
changes.</p>
<div class="section" id="column">
<h2>Column<a class="headerlink" href="#column" title="Permalink to this headline"></a></h2>
<p>Given a standard SQLAlchemy table:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="n">table</span> <span class="o">=</span> <span class="n">Table</span><span class="p">(</span><span class="s">&#39;mytable&#39;</span><span class="p">,</span> <span class="n">meta</span><span class="p">,</span>
    <span class="n">Column</span><span class="p">(</span><span class="s">&#39;id&#39;</span><span class="p">,</span> <span class="n">Integer</span><span class="p">,</span> <span class="n">primary_key</span><span class="o">=</span><span class="bp">True</span><span class="p">),</span>
<span class="p">)</span>
<span class="n">table</span><span class="o">.</span><span class="n">create</span><span class="p">()</span>
</pre></div>
</div>
<p id="column-create">You can create a column with <a class="reference internal" href="api.html#migrate.changeset.schema.ChangesetColumn.create" title="migrate.changeset.schema.ChangesetColumn.create"><tt class="xref py py-meth docutils literal"><span class="pre">create()</span></tt></a>:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="n">col</span> <span class="o">=</span> <span class="n">Column</span><span class="p">(</span><span class="s">&#39;col1&#39;</span><span class="p">,</span> <span class="n">String</span><span class="p">,</span> <span class="n">default</span><span class="o">=</span><span class="s">&#39;foobar&#39;</span><span class="p">)</span>
<span class="n">col</span><span class="o">.</span><span class="n">create</span><span class="p">(</span><span class="n">table</span><span class="p">,</span> <span class="n">populate_default</span><span class="o">=</span><span class="bp">True</span><span class="p">)</span>

<span class="c"># Column is added to table based on its name</span>
<span class="k">assert</span> <span class="n">col</span> <span class="ow">is</span> <span class="n">table</span><span class="o">.</span><span class="n">c</span><span class="o">.</span><span class="n">col1</span>

<span class="c"># col1 is populated with &#39;foobar&#39; because of `populate_default`</span>
</pre></div>
</div>
<div class="admonition note" id="column-drop">
<p class="first admonition-title">Note</p>
<p>You can pass <cite>primary_key_name</cite>, <cite>index_name</cite> and <cite>unique_name</cite> to the
<a class="reference internal" href="api.html#migrate.changeset.schema.ChangesetColumn.create" title="migrate.changeset.schema.ChangesetColumn.create"><tt class="xref py py-meth docutils literal"><span class="pre">create()</span></tt></a> method to issue <tt class="docutils literal"><span class="pre">ALTER</span> <span class="pre">TABLE</span> <span class="pre">ADD</span>
<span class="pre">CONSTRAINT</span></tt> after changing the column.</p>
<p>For multi columns constraints and other advanced configuration, check the
<a class="reference internal" href="#constraint-tutorial"><em>constraint tutorial</em></a>.</p>
<div class="last versionadded">
<p><span class="versionmodified">New in version 0.6.0.</span></p>
</div>
</div>
<p>You can drop a column with <a class="reference internal" href="api.html#migrate.changeset.schema.ChangesetColumn.drop" title="migrate.changeset.schema.ChangesetColumn.drop"><tt class="xref py py-meth docutils literal"><span class="pre">drop()</span></tt></a>:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="n">col</span><span class="o">.</span><span class="n">drop</span><span class="p">()</span>
</pre></div>
</div>
<p id="column-alter">You can alter a column with <a class="reference internal" href="api.html#migrate.changeset.schema.ChangesetColumn.alter" title="migrate.changeset.schema.ChangesetColumn.alter"><tt class="xref py py-meth docutils literal"><span class="pre">alter()</span></tt></a>:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="n">col</span><span class="o">.</span><span class="n">alter</span><span class="p">(</span><span class="n">name</span><span class="o">=</span><span class="s">&#39;col2&#39;</span><span class="p">)</span>

<span class="c"># Renaming a column affects how it&#39;s accessed by the table object</span>
<span class="k">assert</span> <span class="n">col</span> <span class="ow">is</span> <span class="n">table</span><span class="o">.</span><span class="n">c</span><span class="o">.</span><span class="n">col2</span>

<span class="c"># Other properties can be modified as well</span>
<span class="n">col</span><span class="o">.</span><span class="n">alter</span><span class="p">(</span><span class="nb">type</span><span class="o">=</span><span class="n">String</span><span class="p">(</span><span class="mi">42</span><span class="p">),</span> <span class="n">default</span><span class="o">=</span><span class="s">&quot;life, the universe, and everything&quot;</span><span class="p">,</span> <span class="n">nullable</span><span class="o">=</span><span class="bp">False</span><span class="p">)</span>

<span class="c"># Given another column object, col1.alter(col2), col1 will be changed to match col2</span>
<span class="n">col</span><span class="o">.</span><span class="n">alter</span><span class="p">(</span><span class="n">Column</span><span class="p">(</span><span class="s">&#39;col3&#39;</span><span class="p">,</span> <span class="n">String</span><span class="p">(</span><span class="mi">77</span><span class="p">),</span> <span class="n">nullable</span><span class="o">=</span><span class="bp">True</span><span class="p">))</span>
<span class="k">assert</span> <span class="n">col</span><span class="o">.</span><span class="n">nullable</span>
<span class="k">assert</span> <span class="n">table</span><span class="o">.</span><span class="n">c</span><span class="o">.</span><span class="n">col3</span> <span class="ow">is</span> <span class="n">col</span>
</pre></div>
</div>
<div class="deprecated">
<p><span class="versionmodified">Deprecated since version 0.6.0: </span>Passing a <tt class="xref py py-class docutils literal"><span class="pre">Column</span></tt> to
<a class="reference internal" href="api.html#migrate.changeset.schema.ChangesetColumn.alter" title="migrate.changeset.schema.ChangesetColumn.alter"><tt class="xref py py-meth docutils literal"><span class="pre">ChangesetColumn.alter()</span></tt></a> is deprecated. Pass in explicit
parameters, such as <cite>name</cite> for a new column name and <cite>type</cite> for a
new column type, instead. Do <strong>not</strong> include any parameters that
are not changed.</p>
</div>
</div>
<div class="section" id="table">
<span id="table-rename"></span><h2>Table<a class="headerlink" href="#table" title="Permalink to this headline"></a></h2>
<p>SQLAlchemy includes support for <a class="reference external" href="http://www.sqlalchemy.org/docs/core/schema.html#creating-and-dropping-database-tables">creating and dropping</a> tables..</p>
<p>Tables can be renamed with <a class="reference internal" href="api.html#migrate.changeset.schema.ChangesetTable.rename" title="migrate.changeset.schema.ChangesetTable.rename"><tt class="xref py py-meth docutils literal"><span class="pre">rename()</span></tt></a>:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="n">table</span><span class="o">.</span><span class="n">rename</span><span class="p">(</span><span class="s">&#39;newtablename&#39;</span><span class="p">)</span>
</pre></div>
</div>
</div>
<div class="section" id="index">
<span id="index-rename"></span><h2>Index<a class="headerlink" href="#index" title="Permalink to this headline"></a></h2>
<p>SQLAlchemy supports <a class="reference external" href="http://www.sqlalchemy.org/docs/core/schema.html#indexes">creating and dropping</a> indexes.</p>
<p>Indexes can be renamed using
<a class="reference internal" href="api.html#migrate.changeset.schema.ChangesetIndex.rename" title="migrate.changeset.schema.ChangesetIndex.rename"><tt class="xref py py-meth docutils literal"><span class="pre">rename()</span></tt></a>:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="n">index</span><span class="o">.</span><span class="n">rename</span><span class="p">(</span><span class="s">&#39;newindexname&#39;</span><span class="p">)</span>
</pre></div>
</div>
</div>
<div class="section" id="constraint">
<span id="constraint-tutorial"></span><h2>Constraint<a class="headerlink" href="#constraint" title="Permalink to this headline"></a></h2>
<p>SQLAlchemy supports creating or dropping constraints at the same time a table
is created or dropped. SQLAlchemy Migrate adds support for creating and
dropping <tt class="xref py py-class docutils literal"><span class="pre">PrimaryKeyConstraint</span></tt>,
<tt class="xref py py-class docutils literal"><span class="pre">ForeignKeyConstraint</span></tt>,
<tt class="xref py py-class docutils literal"><span class="pre">CheckConstraint</span></tt> and
<tt class="xref py py-class docutils literal"><span class="pre">UniqueConstraint</span></tt> constraints independently using
<tt class="docutils literal"><span class="pre">ALTER</span> <span class="pre">TABLE</span></tt> statements.</p>
<p>The following rundowns are true for all constraints classes:</p>
<ol class="arabic">
<li><p class="first">Make sure you import the relevant constraint class from <a class="reference internal" href="index.html#module-migrate" title="migrate"><tt class="xref py py-mod docutils literal"><span class="pre">migrate</span></tt></a> and
not from <tt class="xref py py-mod docutils literal"><span class="pre">sqlalchemy</span></tt>, for example:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">from</span> <span class="nn">migrate.changeset.constraint</span> <span class="kn">import</span> <span class="n">ForeignKeyConstraint</span>
</pre></div>
</div>
<p>The classes in that module have the extra
<a class="reference internal" href="api.html#migrate.changeset.constraint.ConstraintChangeset.create" title="migrate.changeset.constraint.ConstraintChangeset.create"><tt class="xref py py-meth docutils literal"><span class="pre">create()</span></tt></a> and <a class="reference internal" href="api.html#migrate.changeset.constraint.ConstraintChangeset.drop" title="migrate.changeset.constraint.ConstraintChangeset.drop"><tt class="xref py py-meth docutils literal"><span class="pre">drop()</span></tt></a>
methods.</p>
</li>
<li><p class="first">You can also use constraints as in SQLAlchemy. In this case passing table
argument explicitly is required:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="n">cons</span> <span class="o">=</span> <span class="n">PrimaryKeyConstraint</span><span class="p">(</span><span class="s">&#39;id&#39;</span><span class="p">,</span> <span class="s">&#39;num&#39;</span><span class="p">,</span> <span class="n">table</span><span class="o">=</span><span class="bp">self</span><span class="o">.</span><span class="n">table</span><span class="p">)</span>

<span class="c"># Create the constraint</span>
<span class="n">cons</span><span class="o">.</span><span class="n">create</span><span class="p">()</span>

<span class="c"># Drop the constraint</span>
<span class="n">cons</span><span class="o">.</span><span class="n">drop</span><span class="p">()</span>
</pre></div>
</div>
<p>You can also pass in <tt class="xref py py-class docutils literal"><span class="pre">Column</span></tt> objects (and table
argument can be left out):</p>
<div class="highlight-python"><div class="highlight"><pre><span class="n">cons</span> <span class="o">=</span> <span class="n">PrimaryKeyConstraint</span><span class="p">(</span><span class="n">col1</span><span class="p">,</span> <span class="n">col2</span><span class="p">)</span>
</pre></div>
</div>
</li>
<li><p class="first">Some dialects support <tt class="docutils literal"><span class="pre">CASCADE</span></tt> option when dropping constraints:</p>
<blockquote>
<div><div class="highlight-python"><div class="highlight"><pre><span class="n">cons</span> <span class="o">=</span> <span class="n">PrimaryKeyConstraint</span><span class="p">(</span><span class="n">col1</span><span class="p">,</span> <span class="n">col2</span><span class="p">)</span>

<span class="c"># Create the constraint</span>
<span class="n">cons</span><span class="o">.</span><span class="n">create</span><span class="p">()</span>

<span class="c"># Drop the constraint</span>
<span class="n">cons</span><span class="o">.</span><span class="n">drop</span><span class="p">(</span><span class="n">cascade</span><span class="o">=</span><span class="bp">True</span><span class="p">)</span>
</pre></div>
</div>
</div></blockquote>
</li>
</ol>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p class="last">SQLAlchemy Migrate will try to guess the name of the constraints for
databases, but if it&#8217;s something other than the default, you&#8217;ll need to
give its name. Best practice is to always name your constraints. Note that
Oracle requires that you state the name of the constraint to be created or
dropped.</p>
</div>
<div class="section" id="examples">
<h3>Examples<a class="headerlink" href="#examples" title="Permalink to this headline"></a></h3>
<p>Primary key constraints:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">from</span> <span class="nn">migrate.changeset.constraint</span> <span class="kn">import</span> <span class="n">PrimaryKeyConstraint</span>

<span class="n">cons</span> <span class="o">=</span> <span class="n">PrimaryKeyConstraint</span><span class="p">(</span><span class="n">col1</span><span class="p">,</span> <span class="n">col2</span><span class="p">)</span>

<span class="c"># Create the constraint</span>
<span class="n">cons</span><span class="o">.</span><span class="n">create</span><span class="p">()</span>

<span class="c"># Drop the constraint</span>
<span class="n">cons</span><span class="o">.</span><span class="n">drop</span><span class="p">()</span>
</pre></div>
</div>
<p>Foreign key constraints:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">from</span> <span class="nn">migrate.changeset.constraint</span> <span class="kn">import</span> <span class="n">ForeignKeyConstraint</span>

<span class="n">cons</span> <span class="o">=</span> <span class="n">ForeignKeyConstraint</span><span class="p">([</span><span class="n">table</span><span class="o">.</span><span class="n">c</span><span class="o">.</span><span class="n">fkey</span><span class="p">],</span> <span class="p">[</span><span class="n">othertable</span><span class="o">.</span><span class="n">c</span><span class="o">.</span><span class="n">id</span><span class="p">])</span>

<span class="c"># Create the constraint</span>
<span class="n">cons</span><span class="o">.</span><span class="n">create</span><span class="p">()</span>

<span class="c"># Drop the constraint</span>
<span class="n">cons</span><span class="o">.</span><span class="n">drop</span><span class="p">()</span>
</pre></div>
</div>
<p>Check constraints:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">from</span> <span class="nn">migrate.changeset.constraint</span> <span class="kn">import</span> <span class="n">CheckConstraint</span>

<span class="n">cons</span> <span class="o">=</span> <span class="n">CheckConstraint</span><span class="p">(</span><span class="s">&#39;id &gt; 3&#39;</span><span class="p">,</span> <span class="n">columns</span><span class="o">=</span><span class="p">[</span><span class="n">table</span><span class="o">.</span><span class="n">c</span><span class="o">.</span><span class="n">id</span><span class="p">])</span>

<span class="c"># Create the constraint</span>
<span class="n">cons</span><span class="o">.</span><span class="n">create</span><span class="p">()</span>

<span class="c"># Drop the constraint</span>
<span class="n">cons</span><span class="o">.</span><span class="n">drop</span><span class="p">()</span>
</pre></div>
</div>
<p>Unique constraints:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">from</span> <span class="nn">migrate.changeset.constraint</span> <span class="kn">import</span> <span class="n">UniqueConstraint</span>

<span class="n">cons</span> <span class="o">=</span> <span class="n">UniqueConstraint</span><span class="p">(</span><span class="s">&#39;id&#39;</span><span class="p">,</span> <span class="s">&#39;age&#39;</span><span class="p">,</span> <span class="n">table</span><span class="o">=</span><span class="bp">self</span><span class="o">.</span><span class="n">table</span><span class="p">)</span>

<span class="c"># Create the constraint</span>
<span class="n">cons</span><span class="o">.</span><span class="n">create</span><span class="p">()</span>

<span class="c"># Drop the constraint</span>
<span class="n">cons</span><span class="o">.</span><span class="n">drop</span><span class="p">()</span>
</pre></div>
</div>
</div>
</div>
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