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 MetaData object which contains Table objects defining our database. We make sure this is loaded in env.py and then passed to EnvironmentContext.configure() via the target_metadata argument. The env.py sample script used in the generic template already has a variable declaration near the top for our convenience, where we replace None with our 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

Note

The above example refers to the generic alembic env.py template, e.g. the one created by default when calling upon alembic init, and not the special-use templates such as multidb. Please consult the source code and comments within the env.py script directly for specific guidance on where and how the autogenerate metadata is established.

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

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

    with engine.connect() as connection:
        context.configure(
                    connection=connection,
                    target_metadata=target_metadata
                    )

        with context.begin_transaction():
            context.run_migrations()

We can then use the alembic revision command in conjunction with the --autogenerate option. Suppose our 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 /path/to/foo/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.

What does Autogenerate Detect (and what does it not detect?)

The vast majority of user issues with Alembic centers on the topic of what kinds of changes autogenerate can and cannot detect reliably, as well as how it renders Python code for what it does detect. it is critical to note that autogenerate is not intended to be perfect. It is always necessary to manually review and correct the candidate migrations that autogenererate produces. The feature is getting more and more comprehensive and error-free as releases continue, but one should take note of the current limitations.

Autogenerate will detect:

  • Table additions, removals.
  • Column additions, removals.
  • Change of nullable status on columns.
  • Basic changes in indexes and explcitly-named unique constraints

New in version 0.6.1: Support for autogenerate of indexes and unique constraints.

  • Basic changes in foreign key constraints

New in version 0.7.1: Support for autogenerate of foreign key constraints.

Autogenerate can optionally detect:

  • Change of column type. This will occur if you set the EnvironmentContext.configure.compare_type parameter to True, or to a custom callable function. 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 section Comparing Types for details.
  • Change of server default. This will occur if you set the EnvironmentContext.configure.compare_server_default parameter to True, or to a custom callable function. 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"). See the section The Importance of Naming Constraints for background on how to configure automatic naming schemes for constraints.
  • Special SQLAlchemy types such as 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 sqlalchemy.events.DDLEvents.column_reflect() event to detect when a CHAR (or whatever the target type is) is reflected, and change it to an ENUM (or whatever type is desired) if it is known that that’s the intent of the type. The sqlalchemy.events.DDLEvents.after_parent_attach() can be used within the autogenerate process to intercept and un-attach unwanted CHECK constraints.

Autogenerate can’t currently, but will eventually detect:

  • Some free-standing constraint additions and removals may not be supported, including PRIMARY KEY, EXCLUDE, CHECK; these are not necessarily implemented within the autogenerate detection system and also may not be supported by the supporting SQLAlchemy dialect.
  • Sequence additions, removals - not yet implemented.

Autogenerating Multiple MetaData collections

The target_metadata collection may also be defined as a sequence if an application has multiple MetaData collections involved:

from myapp.mymodel1 import Model1Base
from myapp.mymodel2 import Model2Base
target_metadata = [Model1Base.metadata, Model2Base.metadata]

The sequence of MetaData collections will be consulted in order during the autogenerate process. Note that each MetaData must contain unique table keys (e.g. the “key” is the combination of the table’s name and schema); if two MetaData objects contain a table with the same schema/name combination, an error is raised.

Changed in version 0.9.0: the EnvironmentContext.configure.target_metadata parameter may now be passed a sequence of MetaData objects to support autogeneration of multiple MetaData collections.

Comparing and Rendering Types

The area of autogenerate’s behavior of comparing and rendering Python-based type objects in migration scripts presents a challenge, in that there’s a very wide variety of types to be rendered in scripts, including those part of SQLAlchemy as well as user-defined types. A few options are given to help out with this task.

Controlling the Module Prefix

When types are rendered, they are generated with a module prefix, so that they are available based on a relatively small number of imports. The rules for what the prefix is is based on the kind of datatype as well as configurational settings. For example, when Alembic renders SQLAlchemy types, it will by default prefix the type name with the prefix sa.:

Column("my_column", sa.Integer())

The use of the sa. prefix is controllable by altering the value of EnvironmentContext.configure.sqlalchemy_module_prefix:

def run_migrations_online():
    # ...

    context.configure(
                connection=connection,
                target_metadata=target_metadata,
                sqlalchemy_module_prefix="sqla.",
                # ...
                )

    # ...

In either case, the sa. prefix, or whatever prefix is desired, should also be included in the imports section of script.py.mako; it also defaults to import sqlalchemy as sa.

For user-defined types, that is, any custom type that is not within the sqlalchemy. module namespace, by default Alembic will use the value of __module__ for the custom type:

Column("my_column", myapp.models.utils.types.MyCustomType())

The imports for the above type again must be made present within the migration, either manually, or by adding it to script.py.mako.

Changed in version 0.7.0: The default module prefix rendering for a user-defined type now makes use of the type’s __module__ attribute to retrieve the prefix, rather than using the value of sqlalchemy_module_prefix.

The above custom type has a long and cumbersome name based on the use of __module__ directly, which also implies that lots of imports would be needed in order to accomodate lots of types. For this reason, it is recommended that user-defined types used in migration scripts be made available from a single module. Suppose we call it myapp.migration_types:

# myapp/migration_types.py

from myapp.models.utils.types import MyCustomType

We can first add an import for migration_types to our script.py.mako:

from alembic import op
import sqlalchemy as sa
import myapp.migration_types
${imports if imports else ""}

We then override Alembic’s use of __module__ by providing a fixed prefix, using the EnvironmentContext.configure.user_module_prefix option:

def run_migrations_online():
    # ...

    context.configure(
                connection=connection,
                target_metadata=target_metadata,
                user_module_prefix="myapp.migration_types.",
                # ...
                )

    # ...

Above, we now would get a migration like:

Column("my_column", myapp.migration_types.MyCustomType())

Now, when we inevitably refactor our application to move MyCustomType somewhere else, we only need modify the myapp.migration_types module, instead of searching and replacing all instances within our migration scripts.

Affecting the Rendering of Types Themselves

The methodology Alembic uses to generate SQLAlchemy and user-defined 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. This, in combination with EnvironmentContext.configure.user_module_prefix, is usually enough. However, if additional behaviors are needed, a more comprehensive hook is the EnvironmentContext.configure.render_item option. This hook allows one to provide a callable function within env.py that will fully take over how a type is rendered, including its module prefix:

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,
                # ...
                )

    # ...

In the above example, we’d ensure our MySpecialType includes an appropriate __repr__() method, which is invoked when we call it against "%r".

The callable we use for EnvironmentContext.configure.render_item can also add imports to our migration script. The AutogenContext passed in contains a datamember called AutogenContext.imports, which is a Python set() for which we can add new imports. For example, if MySpecialType were in a module called mymodel.types, we can add the import for it as we encounter the type:

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

    if type_ == 'type' and isinstance(obj, MySpecialType):
        # add import for this type
        autogen_context.imports.add("from mymodel import types")
        return "types.%r" % obj

    # default rendering for other objects
    return False

Changed in version 0.8: The autogen_context data member passed to the render_item callable is now an instance of AutogenContext.

Changed in version 0.8.3: The “imports” data member of the autogen context is restored to the new AutogenContext object as AutogenContext.imports.

The finished migration script will include our imports where the ${imports} expression is used, producing output such as:

from alembic import op
import sqlalchemy as sa
from mymodel import types

def upgrade():
    op.add_column('sometable', Column('mycolumn', types.MySpecialType()))

Comparing Types

The default type comparison logic will work for SQLAlchemy built in types as well as basic user defined types. This logic is only enabled if the EnvironmentContext.configure.compare_type parameter is set to True:

context.configure(
    # ...
    compare_type = True
)

Alternatively, the EnvironmentContext.configure.compare_type parameter accepts a callable function which may be used to implement custom type comparison logic, for cases such as where special user defined types are being used:

def my_compare_type(context, inspected_column,
            metadata_column, inspected_type, metadata_type):
    # return True if the types are different,
    # False if not, or None to allow the default implementation
    # to compare these types
    return None

context.configure(
    # ...
    compare_type = my_compare_type
)

Above, inspected_column is a sqlalchemy.schema.Column as returned by sqlalchemy.engine.reflection.Inspector.reflecttable(), whereas metadata_column is a sqlalchemy.schema.Column from the local model environment. A return value of None indicates that default type comparison to proceed.

Additionally, custom types that are part of imported or third party packages which have special behaviors such as per-dialect behavior should implement a method called compare_against_backend() on their SQLAlchemy type. If this method is present, it will be called where it can also return True or False to specify the types compare as equivalent or not; if it returns None, default type comparison logic will proceed:

class MySpecialType(TypeDecorator):

    # ...

    def compare_against_backend(self, dialect, conn_type):
        # return True if the types are different,
        # False if not, or None to allow the default implementation
        # to compare these types
        if dialect.name == 'postgresql':
            return isinstance(conn_type, postgresql.UUID)
        else:
            return isinstance(conn_type, String)

The order of precedence regarding the EnvironmentContext.configure.compare_type callable vs. the type itself implementing compare_against_backend is that the EnvironmentContext.configure.compare_type callable is favored first; if it returns None, then the compare_against_backend method will be used, if present on the metadata type. If that returns None, then a basic check for type equivalence is run.

New in version 0.7.6: - added support for the compare_against_backend() method.