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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`.

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 to 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 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.

    Alembic includes a set of initialization templates which feature different varieties of env.py for different use cases.

  • README - included with the various enviromnent templates, should have something informative.

  • script.py.mako - This is a Mako 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 yourproject
$ 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.
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 be anywhere, either in the same directory from which the alembic script will normally be invoked, or if in a different directory, can be specified by using the --config option to the alembic runner.

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

# 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

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

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.

  • 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, 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 as returned by datetime.datetime.now()
    .. versionadded:: 0.3.6 - added date parameters to ``file_template``.

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

    .. versionadded:: 0.6.1 - added ``truncate_slug_length`` configuration

  • sqlalchemy.url - A URL to connect to the database via SQLAlchemy. This key is in fact only referenced within the env.py 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.

  • 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 (new in 0.3.3).

  • [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. 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 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: None
Create Date: 2011-11-08 11:40:27.089406

"""

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

Relative Migration Identifiers

As of 0.3.3, 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

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:

$ alembic history

1975ea83b712 -> ae1027a6acf (head), Add a column
None -> 1975ea83b712, empty message

We can also identify specific migrations using just enough characters to uniquely identify them. If we wanted to upgrade directly to ae1027a6acf we could say:

$ alembic upgrade ae1

Alembic will stop and let you know if more than one version starts with that prefix.

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, or various combinations of base, head, currrent to specify the current revision, 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:

.. versionadded:: 0.6.0  ``alembic revision`` now accepts the ``-r`` argument to
   specify specific ranges based on version numbers, symbols, or relative deltas.

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

Auto Generating Migrations

Alembic can view the status of the database and compare against the table metadata in the application, generating the "obvious" migrations based on a comparison. This is achieved using the --autogenerate option to the alembic revision command, which places so-called candidate migrations into our new migrations file. We review and modify these by hand as needed, then proceed normally.

To use autogenerate, we first need to modify our env.py so that it gets access to a table metadata object that contains the target. Suppose our application has a declarative base in myapp.mymodel. This base contains a :class:`~sqlalchemy.schema.MetaData` object which contains :class:`~sqlalchemy.schema.Table` objects defining our database. We make sure this is loaded in env.py and then passed to :meth:`.EnvironmentContext.configure` via the target_metadata argument. The env.py sample script already has a variable declaration near the top for our convenience, where we replace None with our :class:`~sqlalchemy.schema.MetaData`. Starting with:

# add your model's MetaData object here
# for 'autogenerate' support
# from myapp import mymodel
# target_metadata = mymodel.Base.metadata
target_metadata = None

we change to:

from myapp.mymodel import Base
target_metadata = Base.metadata

If we look later in the script, down in run_migrations_online(), we can see the directive passed to :meth:`.EnvironmentContext.configure`:

def run_migrations_online():
    engine = engine_from_config(
                config.get_section(config.config_ini_section), prefix='sqlalchemy.')

    connection = engine.connect()
    context.configure(
                connection=connection,
                target_metadata=target_metadata
                )

    trans = connection.begin()
    try:
        context.run_migrations()
        trans.commit()
    except:
        trans.rollback()
        raise

We can then use the alembic revision command in conjunction with the --autogenerate option. Suppose our :class:`~sqlalchemy.schema.MetaData` contained a definition for the account table, and the database did not. We'd get output like:

$ alembic revision --autogenerate -m "Added account table"
INFO [alembic.context] Detected added table 'account'
Generating /Users/classic/Desktop/tmp/alembic/versions/27c6a30d7c24.py...done

We can then view our file 27c6a30d7c24.py and see that a rudimentary migration is already present:

"""empty message

Revision ID: 27c6a30d7c24
Revises: None
Create Date: 2011-11-08 11:40:27.089406

"""

# revision identifiers, used by Alembic.
revision = '27c6a30d7c24'
down_revision = None

from alembic import op
import sqlalchemy as sa

def upgrade():
    ### commands auto generated by Alembic - please adjust! ###
    op.create_table(
    'account',
    sa.Column('id', sa.Integer()),
    sa.Column('name', sa.String(length=50), nullable=False),
    sa.Column('description', sa.VARCHAR(200)),
    sa.Column('last_transaction_date', sa.DateTime()),
    sa.PrimaryKeyConstraint('id')
    )
    ### end Alembic commands ###

def downgrade():
    ### commands auto generated by Alembic - please adjust! ###
    op.drop_table("account")
    ### end Alembic commands ###

The migration hasn't actually run yet, of course. We do that via the usual upgrade command. We should also go into our migration file and alter it as needed, including adjustments to the directives as well as the addition of other directives which these may be dependent on - specifically data changes in between creates/alters/drops.

Autogenerate will by default detect:

  • Table additions, removals.
  • Column additions, removals.
  • Change of nullable status on columns.
  • Basic changes in indexes and explcitly-named unique constraints
.. versionadded:: 0.6.1 Support for autogenerate of indexes and unique constraints.

Autogenerate can optionally detect:

  • Change of column type. This will occur if you set compare_type=True on :meth:`.EnvironmentContext.configure`. The feature works well in most cases, but is off by default so that it can be tested on the target schema first. It can also be customized by passing a callable here; see the function's documentation for details.
  • Change of server default. This will occur if you set compare_server_default=True on :meth:`.EnvironmentContext.configure`. This feature works well for simple cases but cannot always produce accurate results. The Postgresql backend will actually invoke the "detected" and "metadata" values against the database to determine equivalence. The feature is off by default so that it can be tested on the target schema first. Like type comparison, it can also be customized by passing a callable; see the function's documentation for details.

Autogenerate can not detect:

  • Changes of table name. These will come out as an add/drop of two different tables, and should be hand-edited into a name change instead.
  • Changes of column name. Like table name changes, these are detected as a column add/drop pair, which is not at all the same as a name change.
  • Anonymously named constraints. Give your constraints a name, e.g. UniqueConstraint('col1', 'col2', name="my_name")
  • Special SQLAlchemy types such as :class:`~sqlalchemy.types.Enum` when generated on a backend which doesn't support ENUM directly - this because the representation of such a type in the non-supporting database, i.e. a CHAR+ CHECK constraint, could be any kind of CHAR+CHECK. For SQLAlchemy to determine that this is actually an ENUM would only be a guess, something that's generally a bad idea. To implement your own "guessing" function here, use the :meth:`sqlalchemy.events.DDLEvents.column_reflect` event to alter the SQLAlchemy type passed for certain columns and possibly :meth:`sqlalchemy.events.DDLEvents.after_parent_attach` to intercept unwanted CHECK constraints.

Autogenerate can't currently, but will eventually detect:

  • Some free-standing constraint additions and removals, like CHECK and FOREIGN KEY - these are not fully implemented.
  • Sequence additions, removals - not yet implemented.

Rendering Custom Types in Autogenerate

The methodology Alembic uses to generate SQLAlchemy type constructs as Python code is plain old __repr__(). SQLAlchemy's built-in types for the most part have a __repr__() that faithfully renders a Python-compatible constructor call, but there are some exceptions, particularly in those cases when a constructor accepts arguments that aren't compatible with __repr__(), such as a pickling function.

When building a custom type that will be rendered into a migration script, it is often necessary to explicitly give the type a __repr__() that will faithfully reproduce the constructor for that type. But beyond that, it also is usually necessary to change how the enclosing module or package is rendered as well; this is accomplished using the render_item configuration option:

def render_item(type_, obj, autogen_context):
    """Apply custom rendering for selected items."""

    if type_ == 'type' and isinstance(obj, MySpecialType):
        return "mypackage.%r" % obj

    # default rendering for other objects
    return False

def run_migrations_online():
    # ...

    context.configure(
                connection=connection,
                target_metadata=target_metadata,
                render_item=render_item,
                # ...
                )

    # ...

Above, we also need to make sure our MySpecialType includes an appropriate __repr__() method, which is invoked when we call it against "%r".

Generating SQL Scripts (a.k.a. "Offline Mode")

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 offline mode. 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 --sql option passed to any upgrade or downgrade command. We can, for example, generate a script that revises up to rev ae1027a6acf:

$ 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;

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:

$ alembic upgrade ae1027a6acf --sql > migration.sql

Getting the Start Version

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 alembic_version table to read from.

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 start:end syntax:

$ alembic upgrade 1975ea83b712:ae1027a6acf --sql > migration.sql

The start:end syntax is only allowed in offline mode; in "online" mode, the alembic_version table is always used to get at the current version.

It's also possible to have the env.py 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 alembic_version works:

if context.is_offline_mode():
    version_file = os.path.join(os.path.dirname(config.config_file_name), "version.txt")
    if os.path.exists(version_file):
        current_version = open(version_file).read()
    else:
        current_version = None
    context.configure(dialect_name=engine.name, starting_version=current_version)
    context.run_migrations()
    end_version = context.get_revision_argument()
    if end_version and end_version != current_version:
        open(version_file, 'w').write(end_version)

Writing Migration Scripts to Support Script Generation

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 SELECT statement will not work in --sql 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.

Customizing the Environment

Users of the --sql option are encouraged to hack their env.py files to suit their needs. The env.py script as provided is broken into two sections: run_migrations_online() and run_migrations_offline(). Which function is run is determined at the bottom of the script by reading :meth:`.EnvironmentContext.is_offline_mode`, which basically determines if the --sql flag was enabled.

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 :meth:`.EnvironmentContext.configure` function accepts a parameter output_buffer for this purpose. Below we illustrate this within the run_migrations_offline() function:

from alembic import context
import myapp
import sys

db_1 = myapp.db_1
db_2 = myapp.db_2

def run_migrations_offline():
    """Run migrations *without* a SQL connection."""

    for name, engine, file_ in [
        ("db1", db_1, "db1.sql"),
        ("db2", db_2, "db2.sql"),
    ]:
        context.configure(
                    url=engine.url,
                    transactional_ddl=False,
                    output_buffer=open(file_, 'w'))
        context.execute("-- running migrations for '%s'" % name)
        context.run_migrations(name=name)
        sys.stderr.write("Wrote file '%s'" % file_)

def run_migrations_online():
    """Run migrations *with* a SQL connection."""

    for name, engine in [
        ("db1", db_1),
        ("db2", db_2),
    ]:
        connection = engine.connect()
        context.configure(connection=connection)
        try:
            context.run_migrations(name=name)
            session.commit()
        except:
            session.rollback()
            raise

if context.is_offline_mode():
    run_migrations_offline()
else:
    run_migrations_online()

Working with Branches

A branch describes when a source tree is broken up into two versions representing two independent sets of changes. The challenge of a branch is to merge the branches into a single series of changes. Alembic's GUID-based version number scheme allows branches to be reconciled.

Consider if we merged into our source repository another branch which contained a revision for another table called shopping_cart. This revision was made against our first Alembic revision, the one that generated account. After loading the second source tree in, a new file 27c6a30d7c24.py exists within our versions directory. Both it, as well as ae1027a6acf.py, reference 1975ea83b712 as the "downgrade" revision. To illustrate:

# main source tree:
1975ea83b712 (add account table) -> ae1027a6acf (add a column)

# branched source tree
1975ea83b712 (add account table) -> 27c6a30d7c24 (add shopping cart table)

So above we can see 1975ea83b712 is our branch point. The Alembic command branches illustrates this fact:

$ alembic branches
None -> 1975ea83b712 (branchpoint), add account table
     -> 1975ea83b712 -> 27c6a30d7c24 (head), add shopping cart table
     -> 1975ea83b712 -> ae1027a6acf (head), add a column

History shows it too, illustrating two head entries as well as a branchpoint:

$ alembic history

1975ea83b712 -> 27c6a30d7c24 (head), add shopping cart table

1975ea83b712 -> ae1027a6acf (head), add a column
None -> 1975ea83b712 (branchpoint), add account table

Alembic will also refuse to run any migrations until this is resolved:

$ alembic upgrade head
INFO  [alembic.context] Context class PostgresqlContext.
INFO  [alembic.context] Will assume transactional DDL.
Exception: Only a single head supported so far...

We resolve this branch by editing the files to be in a straight line. In this case we edit 27c6a30d7c24.py to point to ae1027a6acf.py:

"""add shopping cart table

Revision ID: 27c6a30d7c24
Revises: ae1027a6acf  # changed from 1975ea83b712
Create Date: 2011-11-08 13:02:14.212810

"""

# revision identifiers, used by Alembic.
revision = '27c6a30d7c24'
# changed from 1975ea83b712
down_revision = 'ae1027a6acf'

The branches command then shows no branches:

$ alembic branches
$

And the history is similarly linear:

$ alembic history

ae1027a6acf -> 27c6a30d7c24 (head), add shopping cart table
1975ea83b712 -> ae1027a6acf, add a column
None -> 1975ea83b712, add account table

Note

A future command called splice will automate this process.

Building an Up to Date Database from Scratch

There's a theory of database migrations that says that the revisions in existence for a database should be able to go from an entirely blank schema to the finished product, and back again. Alembic can roll this way. Though we think it's kind of overkill, considering that SQLAlchemy itself can emit the full CREATE statements for any given model using :meth:`~sqlalchemy.schema.MetaData.create_all`. If you check out a copy of an application, running this will give you the entire database in one shot, without the need to run through all those migration files, which are instead tailored towards applying incremental changes to an existing database.

Alembic can integrate with a :meth:`~sqlalchemy.schema.MetaData.create_all` script quite easily. After running the create operation, tell Alembic to create a new version table, and to stamp it with the most recent revision (i.e. head):

# inside of a "create the database" script, first create
# tables:
my_metadata.create_all(engine)

# then, load the Alembic configuration and generate the
# version table, "stamping" it with the most recent rev:
from alembic.config import Config
from alembic import command
alembic_cfg = Config("/path/to/yourapp/alembic.ini")
command.stamp(alembic_cfg, "head")

When this approach is used, the application can generate the database using normal SQLAlchemy techniques instead of iterating through hundreds of migration scripts. Now, the purpose of the migration scripts is relegated just to movement between versions on out-of-date databases, not new databases. You can now remove old migration files that are no longer represented on any existing environments.

To prune old migration files, simply delete the files. Then, in the earliest, still-remaining migration file, set down_revision to None:

# replace this:
#down_revision = '290696571ad2'

# with this:
down_revision = None

That file now becomes the "base" of the migration series.