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  <div class="section" id="generating-sql-scripts-a-k-a-offline-mode">
<h1>Generating SQL Scripts (a.k.a. &#8220;Offline Mode&#8221;)<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 -&gt; 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 -&gt; ae1027a6acf
ALTER TABLE account ADD COLUMN last_transaction_date TIMESTAMP WITHOUT TIME ZONE;

INSERT INTO alembic_version (version_num) VALUES (&#39;ae1027a6acf&#39;);

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&#8217;d at first be using output
redirection to generate a script:</p>
<div class="highlight-python"><div class="highlight"><pre>$ alembic upgrade ae1027a6acf --sql &gt; 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&#8217;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 &#8220;version&#8221; 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 &gt; 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 &#8220;online&#8221; 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&#8217;s also possible to have the <code class="docutils literal"><span class="pre">env.py</span></code> script retrieve the &#8220;last&#8221; 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">&quot;version.txt&quot;</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">&#39;w&#39;</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&#8217;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&#8217;s also
important that the Alembic directives, all of which are designed specifically to work
in both &#8220;live execution&#8221; as well as &#8220;offline SQL generation&#8221; 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">&quot;&quot;&quot;Run migrations *without* a SQL connection.&quot;&quot;&quot;</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">&quot;db1&quot;</span><span class="p">,</span> <span class="n">db_1</span><span class="p">,</span> <span class="s">&quot;db1.sql&quot;</span><span class="p">),</span>
        <span class="p">(</span><span class="s">&quot;db2&quot;</span><span class="p">,</span> <span class="n">db_2</span><span class="p">,</span> <span class="s">&quot;db2.sql&quot;</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">&#39;w&#39;</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">&quot;-- running migrations for &#39;</span><span class="si">%s</span><span class="s">&#39;&quot;</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">&quot;Wrote file &#39;</span><span class="si">%s</span><span class="s">&#39;&quot;</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">&quot;&quot;&quot;Run migrations *with* a SQL connection.&quot;&quot;&quot;</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">&quot;db1&quot;</span><span class="p">,</span> <span class="n">db_1</span><span class="p">),</span>
        <span class="p">(</span><span class="s">&quot;db2&quot;</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>
</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. &#8220;Offline Mode&#8221;)</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>

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