========
Tutorial
========

Alembic provides for the creation, management, and invocation of *change management*
scripts for a relational database, using SQLAlchemy as the underlying engine.
This tutorial will provide a full introduction to the theory and usage of this tool.

To begin, make sure Alembic is installed as described at :ref:`installation`.
As stated in the linked document, it is usually preferable that Alembic is
installed in the **same module / Python path as that of the target project**,
usually using a `Python virtual environment
<https://docs.python.org/3/tutorial/venv.html>`_, so that when the ``alembic``
command is run, the Python script which is invoked by ``alembic``,  namely your
project's ``env.py`` script, will have access to your application's models.
This is not strictly necessary in all cases, however in the vast majority of
cases is usually preferred.

The tutorial below assumes the ``alembic`` command line utility is present in
the local path and when invoked, will have access to the same Python module
environment as that of the target project.

The Migration Environment
==========================

Usage of Alembic starts with creation of the *Migration Environment*.  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 ``init`` command of Alembic, and is then customizable to suit the specific
needs of the application.

The structure of this environment, including some generated migration scripts, looks like::

    yourproject/
        alembic/
            env.py
            README
            script.py.mako
            versions/
                3512b954651e_add_account.py
                2b1ae634e5cd_add_order_id.py
                3adcc9a56557_rename_username_field.py

The directory includes these directories/files:

* ``yourproject`` - this is the root of your application's source code, or some directory within it.
* ``alembic`` - 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.
* ``env.py`` - 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 then invoke the migration
  engine, using the connection as a source of database connectivity.

  The ``env.py`` 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 environment 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.

  Alembic includes a set of initialization templates which feature different varieties
  of ``env.py`` for different use cases.
* ``README`` - included with the various environment templates, should have something
  informative.
* ``script.py.mako`` - This is a `Mako <http://www.makotemplates.org>`_ template file which
  is used to generate new migration scripts.   Whatever is here is used to generate new
  files within ``versions/``.   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 ``upgrade()`` and ``downgrade()`` functions.  For example,
  the ``multidb`` environment allows for multiple functions to be generated using a
  naming scheme ``upgrade_engine1()``, ``upgrade_engine2()``.
* ``versions/`` - 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.


Creating an Environment
=======================

With a basic understanding of what the environment is, we can create one using ``alembic init``.
This will create an environment using the "generic" template::

    $ cd /path/to/yourproject
    $ source /path/to/yourproject/.venv/bin/activate   # assuming a local virtualenv
    $ alembic init alembic

Where above, the ``init`` command was called to generate a migrations directory called ``alembic``::

    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.

Alembic also includes other environment templates.  These can be listed out using the ``list_templates``
command::

    $ alembic list_templates
    Available templates:

    generic - Generic single-database configuration.
    async - Generic single-database configuration with an async dbapi.
    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

Editing the .ini File
=====================

Alembic placed a file ``alembic.ini`` into the current directory.  This is a file that the ``alembic``
script looks for when invoked.  This file can exist in a different directory, with the location to it
specified by either the ``--config`` option for the ``alembic`` runner or the ``ALEMBIC_CONFIG``
environment variable (the former takes precedence).

The file generated with the "generic" configuration looks like::

    # 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

    # sys.path path, will be prepended to sys.path if present.
    # defaults to the current working directory.
    # (new in 1.5.5)
    prepend_sys_path = .

    # timezone to use when rendering the date within the migration file
    # as well as the filename.
    # If specified, requires the python-dateutil library that can be
    # installed by adding `alembic[tz]` to the pip requirements
    # string value is passed to dateutil.tz.gettz()
    # leave blank for localtime
    # timezone =

    # 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 ${script_location}/versions.  When using multiple version
    # directories, initial revisions must be specified with --version-path.
    # The path separator used here should be the separator specified by "version_path_separator" below.
    # version_locations = %(here)s/bar:%(here)s/bat:${script_location}/versions

    # version path separator; As mentioned above, this is the character used to split
    # version_locations. The default within new alembic.ini files is "os", which uses os.pathsep.
    # If this key is omitted entirely, it falls back to the legacy behavior of splitting on spaces and/or commas.
    # Valid values for version_path_separator are:
    #
    # version_path_separator = :
    # version_path_separator = ;
    # version_path_separator = space
    version_path_separator = os  # Use os.pathsep. Default configuration used for new projects.

    # the output encoding used when revision files
    # are written from script.py.mako
    # output_encoding = utf-8

    sqlalchemy.url = driver://user:pass@localhost/dbname

    # [post_write_hooks]
    # This section defines scripts or Python functions that are run
    # on newly generated revision scripts.  See the documentation for further
    # detail and examples

    # format using "black" - use the console_scripts runner,
    # against the "black" entrypoint
    # hooks = black
    # black.type = console_scripts
    # black.entrypoint = black
    # black.options = -l 79 REVISION_SCRIPT_FILENAME

    # 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

The file is read using Python's :class:`ConfigParser.SafeConfigParser` object.  The
``%(here)s`` 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.

This file contains the following features:

* ``[alembic]`` - this is the section read by Alembic to determine configuration.  Alembic
  itself does not directly read any other areas of the file.    The name "alembic" can
  be customized using the ``--name`` commandline flag; see :ref:`multiple_environments`
  for a basic example of this.

* ``script_location`` - 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.

  This is the only key required by Alembic in all cases.   The generation
  of the .ini file by the command ``alembic init alembic`` automatically placed the
  directory name ``alembic`` here.   The special variable ``%(here)s`` can also be used,
  as in ``%(here)s/alembic``.

  For support of applications that package themselves into .egg files, the value can
  also be specified as a `package resource
  <https://setuptools.readthedocs.io/en/latest/pkg_resources.html>`_, in which
  case ``resource_filename()`` 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.

* ``file_template`` - this is the naming scheme used to generate new migration files.
  The value present is the default, so is commented out.   Tokens available include:

    * ``%%(rev)s`` - revision id
    * ``%%(slug)s`` - a truncated string derived from the revision message
    * ``%%(year)d``, ``%%(month).2d``, ``%%(day).2d``, ``%%(hour).2d``,
      ``%%(minute).2d``, ``%%(second).2d`` - components of the create date,
      by default ``datetime.datetime.now()`` unless the ``timezone``
      configuration option is also used.

* ``timezone`` - an optional timezone name (e.g. ``UTC``, ``EST5EDT``, etc.)
  that will be applied to the timestamp which renders inside the migration
  file's comment as well as within the filename. This option requires installing
  the ``python-dateutil`` library. If ``timezone`` is specified,
  the create date object is no longer derived from ``datetime.datetime.now()``
  and is instead generated as::

      datetime.datetime.utcnow().replace(
            tzinfo=dateutil.tz.tzutc()
      ).astimezone(
          dateutil.tz.gettz(<timezone>)
      )

* ``truncate_slug_length`` - defaults to 40, the max number of characters
  to include in the "slug" field.

* ``sqlalchemy.url`` - A URL to connect to the database via SQLAlchemy.  This
  configuration value is only used if the ``env.py`` file calls upon them;
  in the "generic" template, the call to
  ``config.get_main_option("sqlalchemy.url")`` in the
  ``run_migrations_offline()`` function and the call to
  ``engine_from_config(prefix="sqlalchemy.")``  in the
  ``run_migrations_online()`` function are where this key is referenced.   If
  the SQLAlchemy URL should come from some other source, such as from
  environment variables or a global registry, or if the migration environment
  makes use of multiple database URLs, the developer is encouraged to alter the
  ``env.py`` file to use whatever methods are appropriate in order to acquire
  the database URL or URLs.

* ``revision_environment`` - this is a flag which when set to the value 'true', will indicate
  that the migration environment script ``env.py`` should be run unconditionally when
  generating new revision files, as well as when running the ``alembic history``
  command.

* ``sourceless`` - 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.

* ``version_locations`` - an optional list of revision file locations, to
  allow revisions to exist in multiple directories simultaneously.
  See :ref:`multiple_bases` for examples.

* ``version_path_separator`` - a separator of ``version_locations`` paths.
  It should be defined if multiple ``version_locations`` is used.
  See :ref:`multiple_bases` for examples.

* ``output_encoding`` - the encoding to use when Alembic writes the
  ``script.py.mako`` file into a new migration file.  Defaults to ``'utf-8'``.

* ``[loggers]``, ``[handlers]``, ``[formatters]``, ``[logger_*]``, ``[handler_*]``,
  ``[formatter_*]`` - these sections are all part of Python's standard logging configuration,
  the mechanics of which are documented at `Configuration File Format <http://docs.python.org/library/logging.config.html#configuration-file-format>`_.
  As is the case with the database connection, these directives are used directly as the
  result of the ``logging.config.fileConfig()`` call present in the
  ``env.py`` script, which you're free to modify.

For starting up with just a single database and the generic configuration, setting up
the SQLAlchemy URL is all that's needed::

    sqlalchemy.url = postgresql://scott:tiger@localhost/test


.. _create_migration:

Create a Migration Script
=========================

With the environment in place we can create a new revision, using ``alembic revision``::

    $ alembic revision -m "create account table"
    Generating /path/to/yourproject/alembic/versions/1975ea83b712_create_accoun
    t_table.py...done

A new file ``1975ea83b712_create_account_table.py`` is generated.  Looking inside the file::

    """create account table

    Revision ID: 1975ea83b712
    Revises:
    Create Date: 2011-11-08 11:40:27.089406

    """

    # revision identifiers, used by Alembic.
    revision = '1975ea83b712'
    down_revision = None
    branch_labels = None

    from alembic import op
    import sqlalchemy as sa

    def upgrade():
        pass

    def downgrade():
        pass

The file contains some header information, identifiers for the current revision
and a "downgrade" revision, an import of basic Alembic directives,
and empty ``upgrade()`` and ``downgrade()`` functions.  Our
job here is to populate the ``upgrade()`` and ``downgrade()`` functions with directives that
will apply a set of changes to our database.    Typically, ``upgrade()`` is required
while ``downgrade()`` is only needed if down-revision capability is desired, though it's
probably a good idea.

Another thing to notice is the ``down_revision`` variable.  This is how Alembic
knows the correct order in which to apply migrations.   When we create the next revision,
the new file's ``down_revision`` identifier would point to this one::

    # revision identifiers, used by Alembic.
    revision = 'ae1027a6acf'
    down_revision = '1975ea83b712'

Every time Alembic runs an operation against the ``versions/`` directory, it reads all
the files in, and composes a list based on how the ``down_revision`` identifiers link together,
with the ``down_revision`` of ``None`` 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 :ref:`building_uptodate` for a description on how to do this, while maintaining
the ability to build the current database fully).

We can then add some directives to our script, suppose adding a new table ``account``::

    def upgrade():
        op.create_table(
            'account',
            sa.Column('id', sa.Integer, primary_key=True),
            sa.Column('name', sa.String(50), nullable=False),
            sa.Column('description', sa.Unicode(200)),
        )

    def downgrade():
        op.drop_table('account')

:meth:`~.Operations.create_table` and :meth:`~.Operations.drop_table` 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 ``env.py`` script.

An overview of all Alembic directives is at :ref:`ops`.

Running our First Migration
===========================

We now want to run our migration.   Assuming our database is totally clean, it's as
yet unversioned.   The ``alembic upgrade`` command will run upgrade operations, proceeding
from the current database revision, in this example ``None``, to the given target revision.
We can specify ``1975ea83b712`` 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 ``head``::

    $ alembic upgrade head
    INFO  [alembic.context] Context class PostgresqlContext.
    INFO  [alembic.context] Will assume transactional DDL.
    INFO  [alembic.context] Running upgrade None -> 1975ea83b712

Wow that rocked!   Note that the information we see on the screen is the result of the
logging configuration set up in ``alembic.ini`` - logging the ``alembic`` stream to the
console (standard error, specifically).

The process which occurred here included that Alembic first checked if the database had
a table called ``alembic_version``, 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 ``head``, which is known to be ``1975ea83b712``.
It then invokes the ``upgrade()`` method in each file to get to the target revision.

Running our Second Migration
=============================

Let's do another one so we have some things to play with.    We again create a revision
file::

    $ alembic revision -m "Add a column"
    Generating /path/to/yourapp/alembic/versions/ae1027a6acf_add_a_column.py...
    done

Let's edit this file and add a new column to the ``account`` table::

    """Add a column

    Revision ID: ae1027a6acf
    Revises: 1975ea83b712
    Create Date: 2011-11-08 12:37:36.714947

    """

    # revision identifiers, used by Alembic.
    revision = 'ae1027a6acf'
    down_revision = '1975ea83b712'

    from alembic import op
    import sqlalchemy as sa

    def upgrade():
        op.add_column('account', sa.Column('last_transaction_date', sa.DateTime))

    def downgrade():
        op.drop_column('account', 'last_transaction_date')

Running again to ``head``::

    $ alembic upgrade head
    INFO  [alembic.context] Context class PostgresqlContext.
    INFO  [alembic.context] Will assume transactional DDL.
    INFO  [alembic.context] Running upgrade 1975ea83b712 -> ae1027a6acf

We've now added the ``last_transaction_date`` column to the database.

Partial Revision Identifiers
=============================

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::

    $ alembic upgrade ae1

Above, we use ``ae1`` to refer to revision ``ae1027a6acf``.
Alembic will stop and let you know if more than one version starts with
that prefix.

.. _relative_migrations:

Relative Migration Identifiers
==============================

Relative upgrades/downgrades are also supported.  To move two versions from
the current, a decimal value "+N" can be supplied::

    $ alembic upgrade +2

Negative values are accepted for downgrades::

    $ alembic downgrade -1

Relative identifiers may also be in terms of a specific revision.  For example,
to upgrade to revision ``ae1027a6acf`` plus two additional steps::

    $ alembic upgrade ae10+2

Getting Information
===================

With a few revisions present we can get some information about the state of things.

First we can view the current revision::

    $ 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

``head`` is displayed only if the revision identifier for this database matches the head revision.

We can also view history with ``alembic history``; the ``--verbose`` option
(accepted by several commands, including ``history``, ``current``, ``heads``
and ``branches``) will show us full information about each revision::

    $ 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

Viewing History Ranges
----------------------

Using the ``-r`` option to ``alembic history``, we can also view various slices
of history.  The ``-r`` argument accepts an argument ``[start]:[end]``, where
either may be a revision number, symbols like ``head``, ``heads`` or
``base``,  ``current`` to specify the current revision(s), as well as negative
relative ranges for ``[start]`` and positive relative ranges for ``[end]``::

  $ alembic history -r1975ea:ae1027

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::

  $ alembic history -r-3:current

View all revisions from 1975 to the head::

  $ alembic history -r1975ea:

Downgrading
===========

We can illustrate a downgrade back to nothing, by calling ``alembic downgrade`` back
to the beginning, which in Alembic is called ``base``::

    $ 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

Back to nothing - and up again::

    $ 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

Next Steps
==========

The vast majority of Alembic environments make heavy use of the
"autogenerate" feature.   Continue onto the next section, :doc:`autogenerate`.


