# 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, like CHECK, PRIMARY KEY - these are not fully implemented.
- Sequence additions, removals - not yet implemented.

## 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.

New in version 0.6.3: Added `EnvironmentContext.configure.user_module_prefix`.

### 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.