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<div class="section" id="generating-sql-scripts-a-k-a-offline-mode">
<h1>Generating SQL Scripts (a.k.a. “Offline Mode”)<a class="headerlink" href="#generating-sql-scripts-a-k-a-offline-mode" title="Permalink to this headline">¶</a></h1>
<p>A major capability of Alembic is to generate migrations as SQL scripts, instead of running
them against the database - this is also referred to as <em>offline mode</em>.
This is a critical feature when working in large organizations
where access to DDL is restricted, and SQL scripts must be handed off to DBAs. Alembic makes
this easy via the <code class="docutils literal"><span class="pre">--sql</span></code> option passed to any <code class="docutils literal"><span class="pre">upgrade</span></code> or <code class="docutils literal"><span class="pre">downgrade</span></code> command. We
can, for example, generate a script that revises up to rev <code class="docutils literal"><span class="pre">ae1027a6acf</span></code>:</p>
<div class="highlight-python"><div class="highlight"><pre>$ alembic upgrade ae1027a6acf --sql
INFO [alembic.context] Context class PostgresqlContext.
INFO [alembic.context] Will assume transactional DDL.
BEGIN;
CREATE TABLE alembic_version (
version_num VARCHAR(32) NOT NULL
);
INFO [alembic.context] Running upgrade None -> 1975ea83b712
CREATE TABLE account (
id SERIAL NOT NULL,
name VARCHAR(50) NOT NULL,
description VARCHAR(200),
PRIMARY KEY (id)
);
INFO [alembic.context] Running upgrade 1975ea83b712 -> ae1027a6acf
ALTER TABLE account ADD COLUMN last_transaction_date TIMESTAMP WITHOUT TIME ZONE;
INSERT INTO alembic_version (version_num) VALUES ('ae1027a6acf');
COMMIT;
</pre></div>
</div>
<p>While the logging configuration dumped to standard error, the actual script was dumped to standard output -
so in the absence of further configuration (described later in this section), we’d at first be using output
redirection to generate a script:</p>
<div class="highlight-python"><div class="highlight"><pre>$ alembic upgrade ae1027a6acf --sql > migration.sql
</pre></div>
</div>
<div class="section" id="getting-the-start-version">
<h2>Getting the Start Version<a class="headerlink" href="#getting-the-start-version" title="Permalink to this headline">¶</a></h2>
<p>Notice that our migration script started at the base - this is the default when using offline
mode, as no database connection is present and there’s no <code class="docutils literal"><span class="pre">alembic_version</span></code> table to read from.</p>
<p>One way to provide a starting version in offline mode is to provide a range to the command line.
This is accomplished by providing the “version” in <code class="docutils literal"><span class="pre">start:end</span></code> syntax:</p>
<div class="highlight-python"><div class="highlight"><pre>$ alembic upgrade 1975ea83b712:ae1027a6acf --sql > migration.sql
</pre></div>
</div>
<p>The <code class="docutils literal"><span class="pre">start:end</span></code> syntax is only allowed in offline mode; in “online” mode, the <code class="docutils literal"><span class="pre">alembic_version</span></code>
table is always used to get at the current version.</p>
<p>It’s also possible to have the <code class="docutils literal"><span class="pre">env.py</span></code> script retrieve the “last” version from
the local environment, such as from a local file. A scheme like this would basically
treat a local file in the same way <code class="docutils literal"><span class="pre">alembic_version</span></code> works:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="k">if</span> <span class="n">context</span><span class="o">.</span><span class="n">is_offline_mode</span><span class="p">():</span>
<span class="n">version_file</span> <span class="o">=</span> <span class="n">os</span><span class="o">.</span><span class="n">path</span><span class="o">.</span><span class="n">join</span><span class="p">(</span><span class="n">os</span><span class="o">.</span><span class="n">path</span><span class="o">.</span><span class="n">dirname</span><span class="p">(</span><span class="n">config</span><span class="o">.</span><span class="n">config_file_name</span><span class="p">),</span> <span class="s">"version.txt"</span><span class="p">)</span>
<span class="k">if</span> <span class="n">os</span><span class="o">.</span><span class="n">path</span><span class="o">.</span><span class="n">exists</span><span class="p">(</span><span class="n">version_file</span><span class="p">):</span>
<span class="n">current_version</span> <span class="o">=</span> <span class="nb">open</span><span class="p">(</span><span class="n">version_file</span><span class="p">)</span><span class="o">.</span><span class="n">read</span><span class="p">()</span>
<span class="k">else</span><span class="p">:</span>
<span class="n">current_version</span> <span class="o">=</span> <span class="bp">None</span>
<span class="n">context</span><span class="o">.</span><span class="n">configure</span><span class="p">(</span><span class="n">dialect_name</span><span class="o">=</span><span class="n">engine</span><span class="o">.</span><span class="n">name</span><span class="p">,</span> <span class="n">starting_rev</span><span class="o">=</span><span class="n">current_version</span><span class="p">)</span>
<span class="n">context</span><span class="o">.</span><span class="n">run_migrations</span><span class="p">()</span>
<span class="n">end_version</span> <span class="o">=</span> <span class="n">context</span><span class="o">.</span><span class="n">get_revision_argument</span><span class="p">()</span>
<span class="k">if</span> <span class="n">end_version</span> <span class="ow">and</span> <span class="n">end_version</span> <span class="o">!=</span> <span class="n">current_version</span><span class="p">:</span>
<span class="nb">open</span><span class="p">(</span><span class="n">version_file</span><span class="p">,</span> <span class="s">'w'</span><span class="p">)</span><span class="o">.</span><span class="n">write</span><span class="p">(</span><span class="n">end_version</span><span class="p">)</span>
</pre></div>
</div>
</div>
<div class="section" id="writing-migration-scripts-to-support-script-generation">
<h2>Writing Migration Scripts to Support Script Generation<a class="headerlink" href="#writing-migration-scripts-to-support-script-generation" title="Permalink to this headline">¶</a></h2>
<p>The challenge of SQL script generation is that the scripts we generate can’t rely upon
any client/server database access. This means a migration script that pulls some rows
into memory via a <code class="docutils literal"><span class="pre">SELECT</span></code> statement will not work in <code class="docutils literal"><span class="pre">--sql</span></code> mode. It’s also
important that the Alembic directives, all of which are designed specifically to work
in both “live execution” as well as “offline SQL generation” mode, are used.</p>
</div>
<div class="section" id="customizing-the-environment">
<h2>Customizing the Environment<a class="headerlink" href="#customizing-the-environment" title="Permalink to this headline">¶</a></h2>
<p>Users of the <code class="docutils literal"><span class="pre">--sql</span></code> option are encouraged to hack their <code class="docutils literal"><span class="pre">env.py</span></code> files to suit their
needs. The <code class="docutils literal"><span class="pre">env.py</span></code> script as provided is broken into two sections: <code class="docutils literal"><span class="pre">run_migrations_online()</span></code>
and <code class="docutils literal"><span class="pre">run_migrations_offline()</span></code>. Which function is run is determined at the bottom of the
script by reading <a class="reference internal" href="api/runtime.html#alembic.runtime.environment.EnvironmentContext.is_offline_mode" title="alembic.runtime.environment.EnvironmentContext.is_offline_mode"><code class="xref py py-meth docutils literal"><span class="pre">EnvironmentContext.is_offline_mode()</span></code></a>, which basically determines if the
<code class="docutils literal"><span class="pre">--sql</span></code> flag was enabled.</p>
<p>For example, a multiple database configuration may want to run through each
database and set the output of the migrations to different named files - the <a class="reference internal" href="api/runtime.html#alembic.runtime.environment.EnvironmentContext.configure" title="alembic.runtime.environment.EnvironmentContext.configure"><code class="xref py py-meth docutils literal"><span class="pre">EnvironmentContext.configure()</span></code></a>
function accepts a parameter <code class="docutils literal"><span class="pre">output_buffer</span></code> for this purpose. Below we illustrate
this within the <code class="docutils literal"><span class="pre">run_migrations_offline()</span></code> function:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">from</span> <span class="nn">alembic</span> <span class="kn">import</span> <span class="n">context</span>
<span class="kn">import</span> <span class="nn">myapp</span>
<span class="kn">import</span> <span class="nn">sys</span>
<span class="n">db_1</span> <span class="o">=</span> <span class="n">myapp</span><span class="o">.</span><span class="n">db_1</span>
<span class="n">db_2</span> <span class="o">=</span> <span class="n">myapp</span><span class="o">.</span><span class="n">db_2</span>
<span class="k">def</span> <span class="nf">run_migrations_offline</span><span class="p">():</span>
<span class="sd">"""Run migrations *without* a SQL connection."""</span>
<span class="k">for</span> <span class="n">name</span><span class="p">,</span> <span class="n">engine</span><span class="p">,</span> <span class="n">file_</span> <span class="ow">in</span> <span class="p">[</span>
<span class="p">(</span><span class="s">"db1"</span><span class="p">,</span> <span class="n">db_1</span><span class="p">,</span> <span class="s">"db1.sql"</span><span class="p">),</span>
<span class="p">(</span><span class="s">"db2"</span><span class="p">,</span> <span class="n">db_2</span><span class="p">,</span> <span class="s">"db2.sql"</span><span class="p">),</span>
<span class="p">]:</span>
<span class="n">context</span><span class="o">.</span><span class="n">configure</span><span class="p">(</span>
<span class="n">url</span><span class="o">=</span><span class="n">engine</span><span class="o">.</span><span class="n">url</span><span class="p">,</span>
<span class="n">transactional_ddl</span><span class="o">=</span><span class="bp">False</span><span class="p">,</span>
<span class="n">output_buffer</span><span class="o">=</span><span class="nb">open</span><span class="p">(</span><span class="n">file_</span><span class="p">,</span> <span class="s">'w'</span><span class="p">))</span>
<span class="n">context</span><span class="o">.</span><span class="n">execute</span><span class="p">(</span><span class="s">"-- running migrations for '</span><span class="si">%s</span><span class="s">'"</span> <span class="o">%</span> <span class="n">name</span><span class="p">)</span>
<span class="n">context</span><span class="o">.</span><span class="n">run_migrations</span><span class="p">(</span><span class="n">name</span><span class="o">=</span><span class="n">name</span><span class="p">)</span>
<span class="n">sys</span><span class="o">.</span><span class="n">stderr</span><span class="o">.</span><span class="n">write</span><span class="p">(</span><span class="s">"Wrote file '</span><span class="si">%s</span><span class="s">'"</span> <span class="o">%</span> <span class="n">file_</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">run_migrations_online</span><span class="p">():</span>
<span class="sd">"""Run migrations *with* a SQL connection."""</span>
<span class="k">for</span> <span class="n">name</span><span class="p">,</span> <span class="n">engine</span> <span class="ow">in</span> <span class="p">[</span>
<span class="p">(</span><span class="s">"db1"</span><span class="p">,</span> <span class="n">db_1</span><span class="p">),</span>
<span class="p">(</span><span class="s">"db2"</span><span class="p">,</span> <span class="n">db_2</span><span class="p">),</span>
<span class="p">]:</span>
<span class="n">connection</span> <span class="o">=</span> <span class="n">engine</span><span class="o">.</span><span class="n">connect</span><span class="p">()</span>
<span class="n">context</span><span class="o">.</span><span class="n">configure</span><span class="p">(</span><span class="n">connection</span><span class="o">=</span><span class="n">connection</span><span class="p">)</span>
<span class="k">try</span><span class="p">:</span>
<span class="n">context</span><span class="o">.</span><span class="n">run_migrations</span><span class="p">(</span><span class="n">name</span><span class="o">=</span><span class="n">name</span><span class="p">)</span>
<span class="n">session</span><span class="o">.</span><span class="n">commit</span><span class="p">()</span>
<span class="k">except</span><span class="p">:</span>
<span class="n">session</span><span class="o">.</span><span class="n">rollback</span><span class="p">()</span>
<span class="k">raise</span>
<span class="k">if</span> <span class="n">context</span><span class="o">.</span><span class="n">is_offline_mode</span><span class="p">():</span>
<span class="n">run_migrations_offline</span><span class="p">()</span>
<span class="k">else</span><span class="p">:</span>
<span class="n">run_migrations_online</span><span class="p">()</span>
</pre></div>
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<h3><a href="index.html">Table Of Contents</a></h3>
<ul>
<li><a class="reference internal" href="#">Generating SQL Scripts (a.k.a. “Offline Mode”)</a><ul>
<li><a class="reference internal" href="#getting-the-start-version">Getting the Start Version</a></li>
<li><a class="reference internal" href="#writing-migration-scripts-to-support-script-generation">Writing Migration Scripts to Support Script Generation</a></li>
<li><a class="reference internal" href="#customizing-the-environment">Customizing the Environment</a></li>
</ul>
</li>
</ul>
<h4>Previous topic</h4>
<p class="topless"><a href="autogenerate.html"
title="previous chapter">Auto Generating Migrations</a></p>
<h4>Next topic</h4>
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title="next chapter">The Importance of Naming Constraints</a></p>
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