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<div class="section" id="tutorial">
<h1>Tutorial<a class="headerlink" href="#tutorial" title="Permalink to this headline">¶</a></h1>
<p><a class="reference external" href="http://bitbucket.org/zzzeek/alembic">Alembic</a> provides for the creation, management, and invocation of <em>change management</em>
scripts for a relational database, using <a class="reference external" href="http://www.sqlalchemy.org">SQLAlchemy</a> as the underlying engine.
This tutorial will provide a full introduction to the theory and usage of this tool.</p>
<p>To begin, make sure Alembic is installed as described at <a class="reference internal" href="front.html#installation"><span>Installation</span></a>.</p>
<div class="section" id="the-migration-environment">
<h2>The Migration Environment<a class="headerlink" href="#the-migration-environment" title="Permalink to this headline">¶</a></h2>
<p>Usage of Alembic starts with creation of the <em>Migration Environment</em>. This is a directory of scripts
that is specific to a particular application. The migration environment is created just once,
and is then maintained along with the application’s source code itself. The environment is
created using the <code class="docutils literal"><span class="pre">init</span></code> command of Alembic, and is then customizable to suit the specific
needs of the application.</p>
<p>The structure of this environment, including some generated migration scripts, looks like:</p>
<div class="highlight-python"><div class="highlight"><pre>yourproject/
alembic/
env.py
README
script.py.mako
versions/
3512b954651e_add_account.py
2b1ae634e5cd_add_order_id.py
3adcc9a56557_rename_username_field.py
</pre></div>
</div>
<p>The directory includes these directories/files:</p>
<ul>
<li><p class="first"><code class="docutils literal"><span class="pre">yourproject</span></code> - this is the root of your application’s source code, or some directory within it.</p>
</li>
<li><p class="first"><code class="docutils literal"><span class="pre">alembic</span></code> - this directory lives within your application’s source tree and is the home of the
migration environment. It can be named anything, and a project that uses multiple databases
may even have more than one.</p>
</li>
<li><p class="first"><code class="docutils literal"><span class="pre">env.py</span></code> - This is a Python script that is run whenever the alembic migration tool is invoked.
At the very least, it contains instructions to configure and generate a SQLAlchemy engine,
procure a connection from that engine along with a transaction, and to then invoke the migration
engine, using the connection as a source of database connectivity.</p>
<p>The <code class="docutils literal"><span class="pre">env.py</span></code> script is part of the generated environment so that the way migrations run
is entirely customizable. The exact specifics of how to connect are here, as well as
the specifics of how the migration enviroment are invoked. The script can be modified
so that multiple engines can be operated upon, custom arguments can be passed into the
migration environment, application-specific libraries and models can be loaded in and
made available.</p>
<p>Alembic includes a set of initialization templates which feature different varieties
of <code class="docutils literal"><span class="pre">env.py</span></code> for different use cases.</p>
</li>
<li><p class="first"><code class="docutils literal"><span class="pre">README</span></code> - included with the various enviromnent templates, should have something
informative.</p>
</li>
<li><p class="first"><code class="docutils literal"><span class="pre">script.py.mako</span></code> - This is a <a class="reference external" href="http://www.makotemplates.org">Mako</a> template file which
is used to generate new migration scripts. Whatever is here is used to generate new
files within <code class="docutils literal"><span class="pre">versions/</span></code>. This is scriptable so that the structure of each migration
file can be controlled, including standard imports to be within each, as well as
changes to the structure of the <code class="docutils literal"><span class="pre">upgrade()</span></code> and <code class="docutils literal"><span class="pre">downgrade()</span></code> functions. For example,
the <code class="docutils literal"><span class="pre">multidb</span></code> environment allows for multiple functions to be generated using a
naming scheme <code class="docutils literal"><span class="pre">upgrade_engine1()</span></code>, <code class="docutils literal"><span class="pre">upgrade_engine2()</span></code>.</p>
</li>
<li><p class="first"><code class="docutils literal"><span class="pre">versions/</span></code> - This directory holds the individual version scripts. Users of other migration
tools may notice that the files here don’t use ascending integers, and instead use a
partial GUID approach. In Alembic, the ordering of version scripts is relative
to directives within the scripts themselves, and it is theoretically possible to “splice” version files
in between others, allowing migration sequences from different branches to be merged,
albeit carefully by hand.</p>
</li>
</ul>
</div>
<div class="section" id="creating-an-environment">
<h2>Creating an Environment<a class="headerlink" href="#creating-an-environment" title="Permalink to this headline">¶</a></h2>
<p>With a basic understanding of what the environment is, we can create one using <code class="docutils literal"><span class="pre">alembic</span> <span class="pre">init</span></code>.
This will create an environment using the “generic” template:</p>
<div class="highlight-python"><div class="highlight"><pre>$ cd yourproject
$ alembic init alembic
</pre></div>
</div>
<p>Where above, the <code class="docutils literal"><span class="pre">init</span></code> command was called to generate a migrations directory called <code class="docutils literal"><span class="pre">alembic</span></code>:</p>
<div class="highlight-python"><div class="highlight"><pre>Creating directory /path/to/yourproject/alembic...done
Creating directory /path/to/yourproject/alembic/versions...done
Generating /path/to/yourproject/alembic.ini...done
Generating /path/to/yourproject/alembic/env.py...done
Generating /path/to/yourproject/alembic/README...done
Generating /path/to/yourproject/alembic/script.py.mako...done
Please edit configuration/connection/logging settings in
'/path/to/yourproject/alembic.ini' before proceeding.
</pre></div>
</div>
<p>Alembic also includes other environment templates. These can be listed out using the <code class="docutils literal"><span class="pre">list_templates</span></code>
command:</p>
<div class="highlight-python"><div class="highlight"><pre>$ alembic list_templates
Available templates:
generic - Generic single-database configuration.
multidb - Rudimentary multi-database configuration.
pylons - Configuration that reads from a Pylons project environment.
Templates are used via the 'init' command, e.g.:
alembic init --template pylons ./scripts
</pre></div>
</div>
</div>
<div class="section" id="editing-the-ini-file">
<h2>Editing the .ini File<a class="headerlink" href="#editing-the-ini-file" title="Permalink to this headline">¶</a></h2>
<p>Alembic placed a file <code class="docutils literal"><span class="pre">alembic.ini</span></code> into the current directory. This is a file that the <code class="docutils literal"><span class="pre">alembic</span></code>
script looks for when invoked. This file can be anywhere, either in the same directory
from which the <code class="docutils literal"><span class="pre">alembic</span></code> script will normally be invoked, or if in a different directory, can
be specified by using the <code class="docutils literal"><span class="pre">--config</span></code> option to the <code class="docutils literal"><span class="pre">alembic</span></code> runner.</p>
<p>The file generated with the “generic” configuration looks like:</p>
<div class="highlight-python"><div class="highlight"><pre># A generic, single database configuration.
[alembic]
# path to migration scripts
script_location = alembic
# template used to generate migration files
# file_template = %%(rev)s_%%(slug)s
# max length of characters to apply to the
# "slug" field
#truncate_slug_length = 40
# set to 'true' to run the environment during
# the 'revision' command, regardless of autogenerate
# revision_environment = false
# set to 'true' to allow .pyc and .pyo files without
# a source .py file to be detected as revisions in the
# versions/ directory
# sourceless = false
# version location specification; this defaults
# to alembic/versions. When using multiple version
# directories, initial revisions must be specified with --version-path
# version_locations = %(here)s/bar %(here)s/bat alembic/versions
# the output encoding used when revision files
# are written from script.py.mako
# output_encoding = utf-8
sqlalchemy.url = driver://user:pass@localhost/dbname
# Logging configuration
[loggers]
keys = root,sqlalchemy,alembic
[handlers]
keys = console
[formatters]
keys = generic
[logger_root]
level = WARN
handlers = console
qualname =
[logger_sqlalchemy]
level = WARN
handlers =
qualname = sqlalchemy.engine
[logger_alembic]
level = INFO
handlers =
qualname = alembic
[handler_console]
class = StreamHandler
args = (sys.stderr,)
level = NOTSET
formatter = generic
[formatter_generic]
format = %(levelname)-5.5s [%(name)s] %(message)s
datefmt = %H:%M:%S
</pre></div>
</div>
<p>The file is read using Python’s <code class="xref py py-class docutils literal"><span class="pre">ConfigParser.SafeConfigParser</span></code> object. The
<code class="docutils literal"><span class="pre">%(here)s</span></code> variable is provided as a substitution variable, which
can be used to produce absolute pathnames to directories and files, as we do above
with the path to the Alembic script location.</p>
<p>This file contains the following features:</p>
<ul>
<li><p class="first"><code class="docutils literal"><span class="pre">[alembic]</span></code> - this is the section read by Alembic to determine configuration. Alembic
itself does not directly read any other areas of the file.</p>
</li>
<li><p class="first"><code class="docutils literal"><span class="pre">script_location</span></code> - this is the location of the Alembic environment. It is normally
specified as a filesystem location, either relative or absolute. If the location is
a relative path, it’s interpreted as relative to the current directory.</p>
<p>This is the only key required by Alembic in all cases. The generation
of the .ini file by the command <code class="docutils literal"><span class="pre">alembic</span> <span class="pre">init</span> <span class="pre">alembic</span></code> automatically placed the
directory name <code class="docutils literal"><span class="pre">alembic</span></code> here. The special variable <code class="docutils literal"><span class="pre">%(here)s</span></code> can also be used,
as in <code class="docutils literal"><span class="pre">%(here)s/alembic</span></code>.</p>
<p>For support of applications that package themselves into .egg files, the value can
also be specified
as a <a class="reference external" href="https://pythonhosted.org/setuptools/pkg_resources.html">package resource</a>, in which
case <code class="docutils literal"><span class="pre">resource_filename()</span></code> is used to find the file (new in 0.2.2). Any non-absolute
URI which contains colons is interpreted here as a resource name, rather than
a straight filename.</p>
</li>
<li><p class="first"><code class="docutils literal"><span class="pre">file_template</span></code> - this is the naming scheme used to generate new migration files.
The value present is the default, so is commented out. Tokens available include:</p>
<blockquote>
<div><ul class="simple">
<li><code class="docutils literal"><span class="pre">%%(rev)s</span></code> - revision id</li>
<li><code class="docutils literal"><span class="pre">%%(slug)s</span></code> - a truncated string derived from the revision message</li>
<li><code class="docutils literal"><span class="pre">%%(year)d</span></code>, <code class="docutils literal"><span class="pre">%%(month).2d</span></code>, <code class="docutils literal"><span class="pre">%%(day).2d</span></code>, <code class="docutils literal"><span class="pre">%%(hour).2d</span></code>,
<code class="docutils literal"><span class="pre">%%(minute).2d</span></code>, <code class="docutils literal"><span class="pre">%%(second).2d</span></code> - components of the create date
as returned by <code class="docutils literal"><span class="pre">datetime.datetime.now()</span></code></li>
</ul>
</div></blockquote>
</li>
<li><p class="first"><code class="docutils literal"><span class="pre">truncate_slug_length</span></code> - defaults to 40, the max number of characters
to include in the “slug” field.</p>
<div class="versionadded">
<p><span class="versionmodified">New in version 0.6.1: </span>- added <code class="docutils literal"><span class="pre">truncate_slug_length</span></code> configuration</p>
</div>
</li>
<li><p class="first"><code class="docutils literal"><span class="pre">sqlalchemy.url</span></code> - A URL to connect to the database via SQLAlchemy. This key is in fact
only referenced within the <code class="docutils literal"><span class="pre">env.py</span></code> file that is specific to the “generic” configuration;
a file that can be customized by the developer. A multiple
database configuration may respond to multiple keys here, or may reference other sections
of the file.</p>
</li>
<li><p class="first"><code class="docutils literal"><span class="pre">revision_environment</span></code> - this is a flag which when set to the value ‘true’, will indicate
that the migration environment script <code class="docutils literal"><span class="pre">env.py</span></code> should be run unconditionally when
generating new revision files</p>
</li>
<li><p class="first"><code class="docutils literal"><span class="pre">sourceless</span></code> - when set to ‘true’, revision files that only exist as .pyc
or .pyo files in the versions directory will be used as versions, allowing
“sourceless” versioning folders. When left at the default of ‘false’,
only .py files are consumed as version files.</p>
<div class="versionadded">
<p><span class="versionmodified">New in version 0.6.4.</span></p>
</div>
</li>
<li><p class="first"><code class="docutils literal"><span class="pre">version_locations</span></code> - an optional list of revision file locations, to
allow revisions to exist in multiple directories simultaneously.
See <a class="reference internal" href="branches.html#multiple-bases"><span>Working with Multiple Bases</span></a> for examples.</p>
<div class="versionadded">
<p><span class="versionmodified">New in version 0.7.0.</span></p>
</div>
</li>
<li><p class="first"><code class="docutils literal"><span class="pre">output_encoding</span></code> - the encoding to use when Alembic writes the
<code class="docutils literal"><span class="pre">script.py.mako</span></code> file into a new migration file. Defaults to <code class="docutils literal"><span class="pre">'utf-8'</span></code>.</p>
<div class="versionadded">
<p><span class="versionmodified">New in version 0.7.0.</span></p>
</div>
</li>
<li><p class="first"><code class="docutils literal"><span class="pre">[loggers]</span></code>, <code class="docutils literal"><span class="pre">[handlers]</span></code>, <code class="docutils literal"><span class="pre">[formatters]</span></code>, <code class="docutils literal"><span class="pre">[logger_*]</span></code>, <code class="docutils literal"><span class="pre">[handler_*]</span></code>,
<code class="docutils literal"><span class="pre">[formatter_*]</span></code> - these sections are all part of Python’s standard logging configuration,
the mechanics of which are documented at <a class="reference external" href="http://docs.python.org/library/logging.config.html#configuration-file-format">Configuration File Format</a>.
As is the case with the database connection, these directives are used directly as the
result of the <code class="docutils literal"><span class="pre">logging.config.fileConfig()</span></code> call present in the
<code class="docutils literal"><span class="pre">env.py</span></code> script, which you’re free to modify.</p>
</li>
</ul>
<p>For starting up with just a single database and the generic configuration, setting up
the SQLAlchemy URL is all that’s needed:</p>
<div class="highlight-python"><div class="highlight"><pre>sqlalchemy.url = postgresql://scott:tiger@localhost/test
</pre></div>
</div>
</div>
<div class="section" id="create-a-migration-script">
<span id="create-migration"></span><h2>Create a Migration Script<a class="headerlink" href="#create-a-migration-script" title="Permalink to this headline">¶</a></h2>
<p>With the environment in place we can create a new revision, using <code class="docutils literal"><span class="pre">alembic</span> <span class="pre">revision</span></code>:</p>
<div class="highlight-python"><div class="highlight"><pre>$ alembic revision -m "create account table"
Generating /path/to/yourproject/alembic/versions/1975ea83b712_create_accoun
t_table.py...done
</pre></div>
</div>
<p>A new file <code class="docutils literal"><span class="pre">1975ea83b712_create_account_table.py</span></code> is generated. Looking inside the file:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="sd">"""create account table</span>
<span class="sd">Revision ID: 1975ea83b712</span>
<span class="sd">Revises:</span>
<span class="sd">Create Date: 2011-11-08 11:40:27.089406</span>
<span class="sd">"""</span>
<span class="c"># revision identifiers, used by Alembic.</span>
<span class="n">revision</span> <span class="o">=</span> <span class="s">'1975ea83b712'</span>
<span class="n">down_revision</span> <span class="o">=</span> <span class="bp">None</span>
<span class="n">branch_labels</span> <span class="o">=</span> <span class="bp">None</span>
<span class="kn">from</span> <span class="nn">alembic</span> <span class="kn">import</span> <span class="n">op</span>
<span class="kn">import</span> <span class="nn">sqlalchemy</span> <span class="kn">as</span> <span class="nn">sa</span>
<span class="k">def</span> <span class="nf">upgrade</span><span class="p">():</span>
<span class="k">pass</span>
<span class="k">def</span> <span class="nf">downgrade</span><span class="p">():</span>
<span class="k">pass</span>
</pre></div>
</div>
<p>The file contains some header information, identifiers for the current revision
and a “downgrade” revision, an import of basic Alembic directives,
and empty <code class="docutils literal"><span class="pre">upgrade()</span></code> and <code class="docutils literal"><span class="pre">downgrade()</span></code> functions. Our
job here is to populate the <code class="docutils literal"><span class="pre">upgrade()</span></code> and <code class="docutils literal"><span class="pre">downgrade()</span></code> functions with directives that
will apply a set of changes to our database. Typically, <code class="docutils literal"><span class="pre">upgrade()</span></code> is required
while <code class="docutils literal"><span class="pre">downgrade()</span></code> is only needed if down-revision capability is desired, though it’s
probably a good idea.</p>
<p>Another thing to notice is the <code class="docutils literal"><span class="pre">down_revision</span></code> variable. This is how Alembic
knows the correct order in which to apply migrations. When we create the next revision,
the new file’s <code class="docutils literal"><span class="pre">down_revision</span></code> identifier would point to this one:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="c"># revision identifiers, used by Alembic.</span>
<span class="n">revision</span> <span class="o">=</span> <span class="s">'ae1027a6acf'</span>
<span class="n">down_revision</span> <span class="o">=</span> <span class="s">'1975ea83b712'</span>
</pre></div>
</div>
<p>Every time Alembic runs an operation against the <code class="docutils literal"><span class="pre">versions/</span></code> directory, it reads all
the files in, and composes a list based on how the <code class="docutils literal"><span class="pre">down_revision</span></code> identifiers link together,
with the <code class="docutils literal"><span class="pre">down_revision</span></code> of <code class="docutils literal"><span class="pre">None</span></code> representing the first file. In theory, if a
migration environment had thousands of migrations, this could begin to add some latency to
startup, but in practice a project should probably prune old migrations anyway
(see the section <a class="reference internal" href="cookbook.html#building-uptodate"><span>Building an Up to Date Database from Scratch</span></a> for a description on how to do this, while maintaining
the ability to build the current database fully).</p>
<p>We can then add some directives to our script, suppose adding a new table <code class="docutils literal"><span class="pre">account</span></code>:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="k">def</span> <span class="nf">upgrade</span><span class="p">():</span>
<span class="n">op</span><span class="o">.</span><span class="n">create_table</span><span class="p">(</span>
<span class="s">'account'</span><span class="p">,</span>
<span class="n">sa</span><span class="o">.</span><span class="n">Column</span><span class="p">(</span><span class="s">'id'</span><span class="p">,</span> <span class="n">sa</span><span class="o">.</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="n">sa</span><span class="o">.</span><span class="n">Column</span><span class="p">(</span><span class="s">'name'</span><span class="p">,</span> <span class="n">sa</span><span class="o">.</span><span class="n">String</span><span class="p">(</span><span class="mi">50</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="n">sa</span><span class="o">.</span><span class="n">Column</span><span class="p">(</span><span class="s">'description'</span><span class="p">,</span> <span class="n">sa</span><span class="o">.</span><span class="n">Unicode</span><span class="p">(</span><span class="mi">200</span><span class="p">)),</span>
<span class="p">)</span>
<span class="k">def</span> <span class="nf">downgrade</span><span class="p">():</span>
<span class="n">op</span><span class="o">.</span><span class="n">drop_table</span><span class="p">(</span><span class="s">'account'</span><span class="p">)</span>
</pre></div>
</div>
<p><a class="reference internal" href="ops.html#alembic.operations.Operations.create_table" title="alembic.operations.Operations.create_table"><code class="xref py py-meth docutils literal"><span class="pre">create_table()</span></code></a> and <a class="reference internal" href="ops.html#alembic.operations.Operations.drop_table" title="alembic.operations.Operations.drop_table"><code class="xref py py-meth docutils literal"><span class="pre">drop_table()</span></code></a> are Alembic directives. Alembic provides
all the basic database migration operations via these directives, which are designed to be as simple and
minimalistic as possible;
there’s no reliance upon existing table metadata for most of these directives. They draw upon
a global “context” that indicates how to get at a database connection (if any; migrations can
dump SQL/DDL directives to files as well) in order to invoke the command. This global
context is set up, like everything else, in the <code class="docutils literal"><span class="pre">env.py</span></code> script.</p>
<p>An overview of all Alembic directives is at <a class="reference internal" href="ops.html#ops"><span>Operation Reference</span></a>.</p>
</div>
<div class="section" id="running-our-first-migration">
<h2>Running our First Migration<a class="headerlink" href="#running-our-first-migration" title="Permalink to this headline">¶</a></h2>
<p>We now want to run our migration. Assuming our database is totally clean, it’s as
yet unversioned. The <code class="docutils literal"><span class="pre">alembic</span> <span class="pre">upgrade</span></code> command will run upgrade operations, proceeding
from the current database revision, in this example <code class="docutils literal"><span class="pre">None</span></code>, to the given target revision.
We can specify <code class="docutils literal"><span class="pre">1975ea83b712</span></code> as the revision we’d like to upgrade to, but it’s easier
in most cases just to tell it “the most recent”, in this case <code class="docutils literal"><span class="pre">head</span></code>:</p>
<div class="highlight-python"><div class="highlight"><pre>$ alembic upgrade head
INFO [alembic.context] Context class PostgresqlContext.
INFO [alembic.context] Will assume transactional DDL.
INFO [alembic.context] Running upgrade None -> 1975ea83b712
</pre></div>
</div>
<p>Wow that rocked! Note that the information we see on the screen is the result of the
logging configuration set up in <code class="docutils literal"><span class="pre">alembic.ini</span></code> - logging the <code class="docutils literal"><span class="pre">alembic</span></code> stream to the
console (standard error, specifically).</p>
<p>The process which occurred here included that Alembic first checked if the database had
a table called <code class="docutils literal"><span class="pre">alembic_version</span></code>, and if not, created it. It looks in this table
for the current version, if any, and then calculates the path from this version to
the version requested, in this case <code class="docutils literal"><span class="pre">head</span></code>, which is known to be <code class="docutils literal"><span class="pre">1975ea83b712</span></code>.
It then invokes the <code class="docutils literal"><span class="pre">upgrade()</span></code> method in each file to get to the target revision.</p>
</div>
<div class="section" id="running-our-second-migration">
<h2>Running our Second Migration<a class="headerlink" href="#running-our-second-migration" title="Permalink to this headline">¶</a></h2>
<p>Let’s do another one so we have some things to play with. We again create a revision
file:</p>
<div class="highlight-python"><div class="highlight"><pre>$ alembic revision -m "Add a column"
Generating /path/to/yourapp/alembic/versions/ae1027a6acf_add_a_column.py...
done
</pre></div>
</div>
<p>Let’s edit this file and add a new column to the <code class="docutils literal"><span class="pre">account</span></code> table:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="sd">"""Add a column</span>
<span class="sd">Revision ID: ae1027a6acf</span>
<span class="sd">Revises: 1975ea83b712</span>
<span class="sd">Create Date: 2011-11-08 12:37:36.714947</span>
<span class="sd">"""</span>
<span class="c"># revision identifiers, used by Alembic.</span>
<span class="n">revision</span> <span class="o">=</span> <span class="s">'ae1027a6acf'</span>
<span class="n">down_revision</span> <span class="o">=</span> <span class="s">'1975ea83b712'</span>
<span class="kn">from</span> <span class="nn">alembic</span> <span class="kn">import</span> <span class="n">op</span>
<span class="kn">import</span> <span class="nn">sqlalchemy</span> <span class="kn">as</span> <span class="nn">sa</span>
<span class="k">def</span> <span class="nf">upgrade</span><span class="p">():</span>
<span class="n">op</span><span class="o">.</span><span class="n">add_column</span><span class="p">(</span><span class="s">'account'</span><span class="p">,</span> <span class="n">sa</span><span class="o">.</span><span class="n">Column</span><span class="p">(</span><span class="s">'last_transaction_date'</span><span class="p">,</span> <span class="n">sa</span><span class="o">.</span><span class="n">DateTime</span><span class="p">))</span>
<span class="k">def</span> <span class="nf">downgrade</span><span class="p">():</span>
<span class="n">op</span><span class="o">.</span><span class="n">drop_column</span><span class="p">(</span><span class="s">'account'</span><span class="p">,</span> <span class="s">'last_transaction_date'</span><span class="p">)</span>
</pre></div>
</div>
<p>Running again to <code class="docutils literal"><span class="pre">head</span></code>:</p>
<div class="highlight-python"><div class="highlight"><pre>$ alembic upgrade head
INFO [alembic.context] Context class PostgresqlContext.
INFO [alembic.context] Will assume transactional DDL.
INFO [alembic.context] Running upgrade 1975ea83b712 -> ae1027a6acf
</pre></div>
</div>
<p>We’ve now added the <code class="docutils literal"><span class="pre">last_transaction_date</span></code> column to the database.</p>
</div>
<div class="section" id="partial-revision-identifiers">
<h2>Partial Revision Identifiers<a class="headerlink" href="#partial-revision-identifiers" title="Permalink to this headline">¶</a></h2>
<p>Any time we need to refer to a revision number explicitly, we have the option
to use a partial number. As long as this number uniquely identifies the
version, it may be used in any command in any place that version numbers
are accepted:</p>
<div class="highlight-python"><div class="highlight"><pre>$ alembic upgrade ae1
</pre></div>
</div>
<p>Above, we use <code class="docutils literal"><span class="pre">ae1</span></code> to refer to revision <code class="docutils literal"><span class="pre">ae1027a6acf</span></code>.
Alembic will stop and let you know if more than one version starts with
that prefix.</p>
</div>
<div class="section" id="relative-migration-identifiers">
<span id="relative-migrations"></span><h2>Relative Migration Identifiers<a class="headerlink" href="#relative-migration-identifiers" title="Permalink to this headline">¶</a></h2>
<p>Relative upgrades/downgrades are also supported. To move two versions from
the current, a decimal value “+N” can be supplied:</p>
<div class="highlight-python"><div class="highlight"><pre>$ alembic upgrade +2
</pre></div>
</div>
<p>Negative values are accepted for downgrades:</p>
<div class="highlight-python"><div class="highlight"><pre>$ alembic downgrade -1
</pre></div>
</div>
<p>Relative identifiers may also be in terms of a specific revision. For example,
to upgrade to revision <code class="docutils literal"><span class="pre">ae1027a6acf</span></code> plus two additional steps:</p>
<div class="highlight-python"><div class="highlight"><pre>$ alembic upgrade ae10+2
</pre></div>
</div>
<div class="versionadded">
<p><span class="versionmodified">New in version 0.7.0: </span>Support for relative migrations in terms of a specific
revision.</p>
</div>
</div>
<div class="section" id="getting-information">
<h2>Getting Information<a class="headerlink" href="#getting-information" title="Permalink to this headline">¶</a></h2>
<p>With a few revisions present we can get some information about the state of things.</p>
<p>First we can view the current revision:</p>
<div class="highlight-python"><div class="highlight"><pre>$ alembic current
INFO [alembic.context] Context class PostgresqlContext.
INFO [alembic.context] Will assume transactional DDL.
Current revision for postgresql://scott:XXXXX@localhost/test: 1975ea83b712 -> ae1027a6acf (head), Add a column
</pre></div>
</div>
<p><code class="docutils literal"><span class="pre">head</span></code> is displayed only if the revision identifier for this database matches the head revision.</p>
<p>We can also view history with <code class="docutils literal"><span class="pre">alembic</span> <span class="pre">history</span></code>; the <code class="docutils literal"><span class="pre">--verbose</span></code> option
(accepted by several commands, including <code class="docutils literal"><span class="pre">history</span></code>, <code class="docutils literal"><span class="pre">current</span></code>, <code class="docutils literal"><span class="pre">heads</span></code>
and <code class="docutils literal"><span class="pre">branches</span></code>) will show us full information about each revision:</p>
<div class="highlight-python"><div class="highlight"><pre>$ alembic history --verbose
Rev: ae1027a6acf (head)
Parent: 1975ea83b712
Path: /path/to/yourproject/alembic/versions/ae1027a6acf_add_a_column.py
add a column
Revision ID: ae1027a6acf
Revises: 1975ea83b712
Create Date: 2014-11-20 13:02:54.849677
Rev: 1975ea83b712
Parent: <base>
Path: /path/to/yourproject/alembic/versions/1975ea83b712_add_account_table.py
create account table
Revision ID: 1975ea83b712
Revises:
Create Date: 2014-11-20 13:02:46.257104
</pre></div>
</div>
<div class="section" id="viewing-history-ranges">
<h3>Viewing History Ranges<a class="headerlink" href="#viewing-history-ranges" title="Permalink to this headline">¶</a></h3>
<p>Using the <code class="docutils literal"><span class="pre">-r</span></code> option to <code class="docutils literal"><span class="pre">alembic</span> <span class="pre">history</span></code>, we can also view various slices
of history. The <code class="docutils literal"><span class="pre">-r</span></code> argument accepts an argument <code class="docutils literal"><span class="pre">[start]:[end]</span></code>, where
either may be a revision number, symbols like <code class="docutils literal"><span class="pre">head</span></code>, <code class="docutils literal"><span class="pre">heads</span></code> or
<code class="docutils literal"><span class="pre">base</span></code>, <code class="docutils literal"><span class="pre">current</span></code> to specify the current revision(s), as well as negative
relative ranges for <code class="docutils literal"><span class="pre">[start]</span></code> and positive relative ranges for <code class="docutils literal"><span class="pre">[end]</span></code>:</p>
<div class="highlight-python"><div class="highlight"><pre>$ alembic history -r1975ea:ae1027
</pre></div>
</div>
<p>A relative range starting from three revs ago up to current migration,
which will invoke the migration environment against the database
to get the current migration:</p>
<div class="highlight-python"><div class="highlight"><pre>$ alembic history -r-3:current
</pre></div>
</div>
<p>View all revisions from 1975 to the head:</p>
<div class="highlight-python"><div class="highlight"><pre>$ alembic history -r1975ea:
</pre></div>
</div>
<div class="versionadded">
<p><span class="versionmodified">New in version 0.6.0: </span><code class="docutils literal"><span class="pre">alembic</span> <span class="pre">revision</span></code> now accepts the <code class="docutils literal"><span class="pre">-r</span></code> argument to
specify specific ranges based on version numbers, symbols, or relative deltas.</p>
</div>
</div>
</div>
<div class="section" id="downgrading">
<h2>Downgrading<a class="headerlink" href="#downgrading" title="Permalink to this headline">¶</a></h2>
<p>We can illustrate a downgrade back to nothing, by calling <code class="docutils literal"><span class="pre">alembic</span> <span class="pre">downgrade</span></code> back
to the beginning, which in Alembic is called <code class="docutils literal"><span class="pre">base</span></code>:</p>
<div class="highlight-python"><div class="highlight"><pre>$ alembic downgrade base
INFO [alembic.context] Context class PostgresqlContext.
INFO [alembic.context] Will assume transactional DDL.
INFO [alembic.context] Running downgrade ae1027a6acf -> 1975ea83b712
INFO [alembic.context] Running downgrade 1975ea83b712 -> None
</pre></div>
</div>
<p>Back to nothing - and up again:</p>
<div class="highlight-python"><div class="highlight"><pre>$ alembic upgrade head
INFO [alembic.context] Context class PostgresqlContext.
INFO [alembic.context] Will assume transactional DDL.
INFO [alembic.context] Running upgrade None -> 1975ea83b712
INFO [alembic.context] Running upgrade 1975ea83b712 -> ae1027a6acf
</pre></div>
</div>
</div>
<div class="section" id="next-steps">
<h2>Next Steps<a class="headerlink" href="#next-steps" title="Permalink to this headline">¶</a></h2>
<p>The vast majority of Alembic environments make heavy use of the
“autogenerate” feature. Continue onto the next section, <a class="reference internal" href="autogenerate.html"><em>Auto Generating Migrations</em></a>.</p>
</div>
</div>
</div>
</div>
</div>
<div class="sphinxsidebar" role="navigation" aria-label="main navigation">
<div class="sphinxsidebarwrapper">
<h3><a href="index.html">Table Of Contents</a></h3>
<ul>
<li><a class="reference internal" href="#">Tutorial</a><ul>
<li><a class="reference internal" href="#the-migration-environment">The Migration Environment</a></li>
<li><a class="reference internal" href="#creating-an-environment">Creating an Environment</a></li>
<li><a class="reference internal" href="#editing-the-ini-file">Editing the .ini File</a></li>
<li><a class="reference internal" href="#create-a-migration-script">Create a Migration Script</a></li>
<li><a class="reference internal" href="#running-our-first-migration">Running our First Migration</a></li>
<li><a class="reference internal" href="#running-our-second-migration">Running our Second Migration</a></li>
<li><a class="reference internal" href="#partial-revision-identifiers">Partial Revision Identifiers</a></li>
<li><a class="reference internal" href="#relative-migration-identifiers">Relative Migration Identifiers</a></li>
<li><a class="reference internal" href="#getting-information">Getting Information</a><ul>
<li><a class="reference internal" href="#viewing-history-ranges">Viewing History Ranges</a></li>
</ul>
</li>
<li><a class="reference internal" href="#downgrading">Downgrading</a></li>
<li><a class="reference internal" href="#next-steps">Next Steps</a></li>
</ul>
</li>
</ul>
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title="next chapter">Auto Generating Migrations</a></p>
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