Cookbook

A collection of “How-Tos”, highlighting various ways to extend Alembic.

Note

This is a new section where we hope to start cataloguing various “how-tos” we come up with based on user requests. It is often the case that users will request a feature only to learn that simple customization can provide the same thing. There’s only one recipe at the moment but we hope to get more soon!

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 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 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 Now, the purpose of the migration scripts is relegated just to movement between versions on out-of-date databases, not new databases.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.

Conditional Migration Elements

This example features the basic idea of a common need, that of affecting how a migration runs based on command line switches.

The technique to use here is simple; within a migration script, inspect the EnvironmentContext.get_x_argument() collection for any additional, user-defined parameters. Then take action based on the presence of those arguments.

To make it such that the logic to inspect these flags is easy to use and modify, we modify our script.py.mako template to make this feature available in all new revision files:

"""${message}

Revision ID: ${up_revision}
Revises: ${down_revision}
Create Date: ${create_date}

"""

# revision identifiers, used by Alembic.
revision = ${repr(up_revision)}
down_revision = ${repr(down_revision)}

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

from alembic import context


def upgrade():
    schema_upgrades()
    if context.get_x_argument(as_dictionary=True).get('data', None):
        data_upgrades()

def downgrade():
    if context.get_x_argument(as_dictionary=True).get('data', None):
        data_downgrades()
    schema_downgrades()

def schema_upgrades():
    """schema upgrade migrations go here."""
    ${upgrades if upgrades else "pass"}

def schema_downgrades():
    """schema downgrade migrations go here."""
    ${downgrades if downgrades else "pass"}

def data_upgrades():
    """Add any optional data upgrade migrations here!"""
    pass

def data_downgrades():
    """Add any optional data downgrade migrations here!"""
    pass

Now, when we create a new migration file, the data_upgrades() and data_downgrades() placeholders will be available, where we can add optional data migrations:

"""rev one

Revision ID: 3ba2b522d10d
Revises: None
Create Date: 2014-03-04 18:05:36.992867

"""

# revision identifiers, used by Alembic.
revision = '3ba2b522d10d'
down_revision = None

from alembic import op
import sqlalchemy as sa
from sqlalchemy import String, Column
from sqlalchemy.sql import table, column

from alembic import context

def upgrade():
    schema_upgrades()
    if context.get_x_argument(as_dictionary=True).get('data', None):
        data_upgrades()

def downgrade():
    if context.get_x_argument(as_dictionary=True).get('data', None):
        data_downgrades()
    schema_downgrades()

def schema_upgrades():
    """schema upgrade migrations go here."""
    op.create_table("my_table", Column('data', String))

def schema_downgrades():
    """schema downgrade migrations go here."""
    op.drop_table("my_table")

def data_upgrades():
    """Add any optional data upgrade migrations here!"""

    my_table = table('my_table',
        column('data', String),
    )

    op.bulk_insert(my_table,
        [
            {'data': 'data 1'},
            {'data': 'data 2'},
            {'data': 'data 3'},
        ]
    )

def data_downgrades():
    """Add any optional data downgrade migrations here!"""

    op.execute("delete from my_table")

To invoke our migrations with data included, we use the -x flag:

alembic -x data=true upgrade head

The EnvironmentContext.get_x_argument() is an easy way to support new commandline options within environment and migration scripts.

Sharing a Connection with a Series of Migration Commands and Environments

It is often the case that an application will need to call upon a series of commands within Commands, where it would be advantageous for all operations to proceed along a single transaction. The connectivity for a migration is typically solely determined within the env.py script of a migration environment, which is called within the scope of a command.

The steps to take here are:

1. Produce the Connection object to use.
2. Place it somewhere that env.py will be able to access it. This can be either a. a module-level global somewhere, or b. an attribute which we place into the Config.attributes dictionary (if we are on an older Alembic version, we may also attach an attribute directly to the Config object).
3. The env.py script is modified such that it looks for this Connection and makes use of it, in lieu of building up its own Engine instance.

We illustrate using Config.attributes:

from alembic import command, config

cfg = config.Config("/path/to/yourapp/alembic.ini")
with engine.begin() as connection:
    cfg.attributes['connection'] = connection
    command.upgrade(cfg, "head")

Then in env.py:

def run_migrations_online():
    connectable = config.attributes.get('connection', None)

    if connectable is None:
        # only create Engine if we don't have a Connection
        # from the outside
        connectable = engine_from_config(
            config.get_section(config.config_ini_section),
            prefix='sqlalchemy.',
            poolclass=pool.NullPool)

    # when connectable is already a Connection object, calling
    # connect() gives us a *branched connection*.

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

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

Branched Connections

Note that we are calling the connect() method, even if we are using a Connection object to start with. The effect this has when calling connect() is that SQLAlchemy passes us a branch of the original connection; it is in every way the same as the Connection we started with, except it provides nested scope; the context we have here as well as the close() method of this branched connection doesn’t actually close the outer connection, which stays active for continued use.

New in version 0.7.5: Added Config.attributes.

Replaceable Objects

This recipe proposes a hypothetical way of dealing with what we might call a replaceable schema object. A replaceable object is a schema object that needs to be created and dropped all at once. Examples of such objects include views, stored procedures, and triggers.

Replaceable objects present a problem in that in order to make incremental changes to them, we have to refer to the whole definition at once. If we need to add a new column to a view, for example, we have to drop it entirely and recreate it fresh with the extra column added, referring to the whole structure; but to make it even tougher, if we wish to support downgrade operarations in our migration scripts, we need to refer to the previous version of that construct fully, and we’d much rather not have to type out the whole definition in multiple places.

This recipe proposes that we may refer to the older version of a replaceable construct by directly naming the migration version in which it was created, and having a migration refer to that previous file as migrations run. We will also demonstrate how to integrate this logic within the Operation Plugins feature introduced in Alembic 0.8. It may be very helpful to review this section first to get an overview of this API.

The Replaceable Object Structure

We first need to devise a simple format that represents the “CREATE XYZ” / “DROP XYZ” aspect of what it is we’re building. We will work with an object that represents a textual definition; while a SQL view is an object that we can define using a table-metadata-like system, this is not so much the case for things like stored procedures, where we pretty much need to have a full string definition written down somewhere. We’ll use a simple value object called ReplaceableObject that can represent any named set of SQL text to send to a “CREATE” statement of some kind:

class ReplaceableObject(object):
    def __init__(self, name, sqltext):
        self.name = name
        self.sqltext = sqltext

Using this object in a migration script, assuming a Postgresql-style syntax, looks like:

customer_view = ReplaceableObject(
    "customer_view",
    "SELECT name, order_count FROM customer WHERE order_count > 0"
)

add_customer_sp = ReplaceableObject(
    "add_customer_sp(name varchar, order_count integer)",
    """
    RETURNS integer AS $$
    BEGIN
        insert into customer (name, order_count)
        VALUES (in_name, in_order_count);
    END;
    $$ LANGUAGE plpgsql;
    """
)

The ReplaceableObject class is only one very simplistic way to do this. The structure of how we represent our schema objects is not too important for the purposes of this example; we can just as well put strings inside of tuples or dictionaries, as well as that we could define any kind of series of fields and class structures we want. The only important part is that below we will illustrate how organize the code that can consume the structure we create here.

Create Operations for the Target Objects

We’ll use the Operations extension API to make new operations for create, drop, and replace of views and stored procedures. Using this API is also optional; we can just as well make any kind of Python function that we would invoke from our migration scripts. However, using this API gives us operations built directly into the Alembic op.* namespace very nicely.

The most intricate class is below. This is the base of our “replaceable” operation, which includes not just a base operation for emitting CREATE and DROP instructions on a ReplaceableObject, it also assumes a certain model of “reversibility” which makes use of references to other migration files in order to refer to the “previous” version of an object:

from alembic.operations import Operations, MigrateOperation

class ReversibleOp(MigrateOperation):
    def __init__(self, target):
        self.target = target

    @classmethod
    def invoke_for_target(cls, operations, target):
        op = cls(target)
        return operations.invoke(op)

    def reverse(self):
        raise NotImplementedError()

    @classmethod
    def _get_object_from_version(cls, operations, ident):
        version, objname = ident.split(".")

        module = operations.get_context().script.get_revision(version).module
        obj = getattr(module, objname)
        return obj

    @classmethod
    def replace(cls, operations, target, replaces=None, replace_with=None):

        if replaces:
            old_obj = cls._get_object_from_version(operations, replaces)
            drop_old = cls(old_obj).reverse()
            create_new = cls(target)
        elif replace_with:
            old_obj = cls._get_object_from_version(operations, replace_with)
            drop_old = cls(target).reverse()
            create_new = cls(old_obj)
        else:
            raise TypeError("replaces or replace_with is required")

        operations.invoke(drop_old)
        operations.invoke(create_new)

The workings of this class should become clear as we walk through the example. To create usable operations from this base, we will build a series of stub classes and use Operations.register_operation() to make them part of the op.* namespace:

@Operations.register_operation("create_view", "invoke_for_target")
@Operations.register_operation("replace_view", "replace")
class CreateViewOp(ReversibleOp):
    def reverse(self):
        return DropViewOp(self.target)


@Operations.register_operation("drop_view", "invoke_for_target")
class DropViewOp(ReversibleOp):
    def reverse(self):
        return CreateViewOp(self.view)


@Operations.register_operation("create_sp", "invoke_for_target")
@Operations.register_operation("replace_sp", "replace")
class CreateSPOp(ReversibleOp):
    def reverse(self):
        return DropSPOp(self.target)


@Operations.register_operation("drop_sp", "invoke_for_target")
class DropSPOp(ReversibleOp):
    def reverse(self):
        return CreateSPOp(self.target)

To actually run the SQL like “CREATE VIEW” and “DROP SEQUENCE”, we’ll provide implementations using Operations.implementation_for() that run straight into Operations.execute():

@Operations.implementation_for(CreateViewOp)
def create_view(operations, operation):
    operations.execute("CREATE VIEW %s AS %s" % (
        operation.target.name,
        operation.target.sqltext
    ))


@Operations.implementation_for(DropViewOp)
def drop_view(operations, operation):
    operations.execute("DROP VIEW %s" % operation.target.name)


@Operations.implementation_for(CreateSPOp)
def create_sp(operations, operation):
    operations.execute(
        "CREATE FUNCTION %s %s" % (
            operation.target.name, operation.target.sqltext
        )
    )


@Operations.implementation_for(DropSPOp)
def drop_sp(operations, operation):
    operations.execute("DROP FUNCTION %s" % operation.target.name)

All of the above code can be present anywhere within an application’s source tree; the only requirement is that when the env.py script is invoked, it includes imports that ultimately call upon these classes as well as the Operations.register_operation() and Operations.implementation_for() sequences.

Create Initial Migrations

We can now illustrate how these objects look during use. For the first step, we’ll create a new migration to create a “customer” table:

$ alembic revision -m "create table"

We build the first revision as follows:

"""create table

Revision ID: 3ab8b2dfb055
Revises:
Create Date: 2015-07-27 16:22:44.918507

"""

# revision identifiers, used by Alembic.
revision = '3ab8b2dfb055'
down_revision = None
branch_labels = None
depends_on = None

from alembic import op
import sqlalchemy as sa


def upgrade():
    op.create_table(
        "customer",
        sa.Column('id', sa.Integer, primary_key=True),
        sa.Column('name', sa.String),
        sa.Column('order_count', sa.Integer),
    )


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

For the second migration, we will create a view and a stored procedure which act upon this table:

$ alembic revision -m "create views/sp"

This migration will use the new directives:

"""create views/sp

Revision ID: 28af9800143f
Revises: 3ab8b2dfb055
Create Date: 2015-07-27 16:24:03.589867

"""

# revision identifiers, used by Alembic.
revision = '28af9800143f'
down_revision = '3ab8b2dfb055'
branch_labels = None
depends_on = None

from alembic import op
import sqlalchemy as sa

from foo import ReplaceableObject

customer_view = ReplaceableObject(
    "customer_view",
    "SELECT name, order_count FROM customer WHERE order_count > 0"
)

add_customer_sp = ReplaceableObject(
    "add_customer_sp(name varchar, order_count integer)",
    """
    RETURNS integer AS $$
    BEGIN
        insert into customer (name, order_count)
        VALUES (in_name, in_order_count);
    END;
    $$ LANGUAGE plpgsql;
    """
)


def upgrade():
    op.create_view(customer_view)
    op.create_sp(add_customer_sp)


def downgrade():
    op.drop_view(customer_view)
    op.drop_sp(add_customer_sp)

We see the use of our new create_view(), create_sp(), drop_view(), and drop_sp() directives. Running these to “head” we get the following (this includes an edited view of SQL emitted):

$ alembic upgrade 28af9800143
INFO  [alembic.runtime.migration] Context impl PostgresqlImpl.
INFO  [alembic.runtime.migration] Will assume transactional DDL.
INFO  [sqlalchemy.engine.base.Engine] BEGIN (implicit)
INFO  [sqlalchemy.engine.base.Engine] select relname from pg_class c join pg_namespace n on n.oid=c.relnamespace where pg_catalog.pg_table_is_visible(c.oid) and relname=%(name)s
INFO  [sqlalchemy.engine.base.Engine] {'name': u'alembic_version'}
INFO  [sqlalchemy.engine.base.Engine] SELECT alembic_version.version_num
FROM alembic_version
INFO  [sqlalchemy.engine.base.Engine] {}
INFO  [sqlalchemy.engine.base.Engine] select relname from pg_class c join pg_namespace n on n.oid=c.relnamespace where pg_catalog.pg_table_is_visible(c.oid) and relname=%(name)s
INFO  [sqlalchemy.engine.base.Engine] {'name': u'alembic_version'}
INFO  [alembic.runtime.migration] Running upgrade  -> 3ab8b2dfb055, create table
INFO  [sqlalchemy.engine.base.Engine]
CREATE TABLE customer (
    id SERIAL NOT NULL,
    name VARCHAR,
    order_count INTEGER,
    PRIMARY KEY (id)
)


INFO  [sqlalchemy.engine.base.Engine] {}
INFO  [sqlalchemy.engine.base.Engine] INSERT INTO alembic_version (version_num) VALUES ('3ab8b2dfb055')
INFO  [sqlalchemy.engine.base.Engine] {}
INFO  [alembic.runtime.migration] Running upgrade 3ab8b2dfb055 -> 28af9800143f, create views/sp
INFO  [sqlalchemy.engine.base.Engine] CREATE VIEW customer_view AS SELECT name, order_count FROM customer WHERE order_count > 0
INFO  [sqlalchemy.engine.base.Engine] {}
INFO  [sqlalchemy.engine.base.Engine] CREATE FUNCTION add_customer_sp(name varchar, order_count integer)
    RETURNS integer AS $$
    BEGIN
        insert into customer (name, order_count)
        VALUES (in_name, in_order_count);
    END;
    $$ LANGUAGE plpgsql;

INFO  [sqlalchemy.engine.base.Engine] {}
INFO  [sqlalchemy.engine.base.Engine] UPDATE alembic_version SET version_num='28af9800143f' WHERE alembic_version.version_num = '3ab8b2dfb055'
INFO  [sqlalchemy.engine.base.Engine] {}
INFO  [sqlalchemy.engine.base.Engine] COMMIT

We see that our CREATE TABLE proceeded as well as the CREATE VIEW and CREATE FUNCTION operations produced by our new directives.

Create Revision Migrations

Finally, we can illustrate how we would “revise” these objects. Let’s consider we added a new column email to our customer table:

$ alembic revision -m "add email col"

The migration is:

"""add email col

Revision ID: 191a2d20b025
Revises: 28af9800143f
Create Date: 2015-07-27 16:25:59.277326

"""

# revision identifiers, used by Alembic.
revision = '191a2d20b025'
down_revision = '28af9800143f'
branch_labels = None
depends_on = None

from alembic import op
import sqlalchemy as sa


def upgrade():
    op.add_column("customer", sa.Column("email", sa.String()))


def downgrade():
    op.drop_column("customer", "email")

We now need to recreate the customer_view view and the add_customer_sp function. To include downgrade capability, we will need to refer to the previous version of the construct; the replace_view() and replace_sp() operations we’ve created make this possible, by allowing us to refer to a specific, previous revision. the replaces and replace_with arguments accept a dot-separated string, which refers to a revision number and an object name, such as "28af9800143f.customer_view". The ReversibleOp class makes use of the Operations.get_context() method to locate the version file we refer to:

$ alembic revision -m "update views/sp"

The migration:

"""update views/sp

Revision ID: 199028bf9856
Revises: 191a2d20b025
Create Date: 2015-07-27 16:26:31.344504

"""

# revision identifiers, used by Alembic.
revision = '199028bf9856'
down_revision = '191a2d20b025'
branch_labels = None
depends_on = None

from alembic import op
import sqlalchemy as sa

from foo import ReplaceableObject

customer_view = ReplaceableObject(
    "customer_view",
    "SELECT name, order_count, email "
    "FROM customer WHERE order_count > 0"
)

add_customer_sp = ReplaceableObject(
    "add_customer_sp(name varchar, order_count integer, email varchar)",
    """
    RETURNS integer AS $$
    BEGIN
        insert into customer (name, order_count, email)
        VALUES (in_name, in_order_count, email);
    END;
    $$ LANGUAGE plpgsql;
    """
)


def upgrade():
    op.replace_view(customer_view, replaces="28af9800143f.customer_view")
    op.replace_sp(add_customer_sp, replaces="28af9800143f.add_customer_sp")


def downgrade():
    op.replace_view(customer_view, replace_with="28af9800143f.customer_view")
    op.replace_sp(add_customer_sp, replace_with="28af9800143f.add_customer_sp")

Above, instead of using create_view(), create_sp(), drop_view(), and drop_sp() methods, we now use replace_view() and replace_sp(). The replace operation we’ve built always runs a DROP and a CREATE. Running an upgrade to head we see:

$ alembic upgrade head
INFO  [alembic.runtime.migration] Context impl PostgresqlImpl.
INFO  [alembic.runtime.migration] Will assume transactional DDL.
INFO  [sqlalchemy.engine.base.Engine] BEGIN (implicit)
INFO  [sqlalchemy.engine.base.Engine] select relname from pg_class c join pg_namespace n on n.oid=c.relnamespace where pg_catalog.pg_table_is_visible(c.oid) and relname=%(name)s
INFO  [sqlalchemy.engine.base.Engine] {'name': u'alembic_version'}
INFO  [sqlalchemy.engine.base.Engine] SELECT alembic_version.version_num
FROM alembic_version
INFO  [sqlalchemy.engine.base.Engine] {}
INFO  [alembic.runtime.migration] Running upgrade 28af9800143f -> 191a2d20b025, add email col
INFO  [sqlalchemy.engine.base.Engine] ALTER TABLE customer ADD COLUMN email VARCHAR
INFO  [sqlalchemy.engine.base.Engine] {}
INFO  [sqlalchemy.engine.base.Engine] UPDATE alembic_version SET version_num='191a2d20b025' WHERE alembic_version.version_num = '28af9800143f'
INFO  [sqlalchemy.engine.base.Engine] {}
INFO  [alembic.runtime.migration] Running upgrade 191a2d20b025 -> 199028bf9856, update views/sp
INFO  [sqlalchemy.engine.base.Engine] DROP VIEW customer_view
INFO  [sqlalchemy.engine.base.Engine] {}
INFO  [sqlalchemy.engine.base.Engine] CREATE VIEW customer_view AS SELECT name, order_count, email FROM customer WHERE order_count > 0
INFO  [sqlalchemy.engine.base.Engine] {}
INFO  [sqlalchemy.engine.base.Engine] DROP FUNCTION add_customer_sp(name varchar, order_count integer)
INFO  [sqlalchemy.engine.base.Engine] {}
INFO  [sqlalchemy.engine.base.Engine] CREATE FUNCTION add_customer_sp(name varchar, order_count integer, email varchar)
    RETURNS integer AS $$
    BEGIN
        insert into customer (name, order_count, email)
        VALUES (in_name, in_order_count, email);
    END;
    $$ LANGUAGE plpgsql;

INFO  [sqlalchemy.engine.base.Engine] {}
INFO  [sqlalchemy.engine.base.Engine] UPDATE alembic_version SET version_num='199028bf9856' WHERE alembic_version.version_num = '191a2d20b025'
INFO  [sqlalchemy.engine.base.Engine] {}
INFO  [sqlalchemy.engine.base.Engine] COMMIT

After adding our new email column, we see that both customer_view and add_customer_sp() are dropped before the new version is created. If we downgrade back to the old version, we see the old version of these recreated again within the downgrade for this migration:

$ alembic downgrade 28af9800143
INFO  [alembic.runtime.migration] Context impl PostgresqlImpl.
INFO  [alembic.runtime.migration] Will assume transactional DDL.
INFO  [sqlalchemy.engine.base.Engine] BEGIN (implicit)
INFO  [sqlalchemy.engine.base.Engine] select relname from pg_class c join pg_namespace n on n.oid=c.relnamespace where pg_catalog.pg_table_is_visible(c.oid) and relname=%(name)s
INFO  [sqlalchemy.engine.base.Engine] {'name': u'alembic_version'}
INFO  [sqlalchemy.engine.base.Engine] SELECT alembic_version.version_num
FROM alembic_version
INFO  [sqlalchemy.engine.base.Engine] {}
INFO  [alembic.runtime.migration] Running downgrade 199028bf9856 -> 191a2d20b025, update views/sp
INFO  [sqlalchemy.engine.base.Engine] DROP VIEW customer_view
INFO  [sqlalchemy.engine.base.Engine] {}
INFO  [sqlalchemy.engine.base.Engine] CREATE VIEW customer_view AS SELECT name, order_count FROM customer WHERE order_count > 0
INFO  [sqlalchemy.engine.base.Engine] {}
INFO  [sqlalchemy.engine.base.Engine] DROP FUNCTION add_customer_sp(name varchar, order_count integer, email varchar)
INFO  [sqlalchemy.engine.base.Engine] {}
INFO  [sqlalchemy.engine.base.Engine] CREATE FUNCTION add_customer_sp(name varchar, order_count integer)
    RETURNS integer AS $$
    BEGIN
        insert into customer (name, order_count)
        VALUES (in_name, in_order_count);
    END;
    $$ LANGUAGE plpgsql;

INFO  [sqlalchemy.engine.base.Engine] {}
INFO  [sqlalchemy.engine.base.Engine] UPDATE alembic_version SET version_num='191a2d20b025' WHERE alembic_version.version_num = '199028bf9856'
INFO  [sqlalchemy.engine.base.Engine] {}
INFO  [alembic.runtime.migration] Running downgrade 191a2d20b025 -> 28af9800143f, add email col
INFO  [sqlalchemy.engine.base.Engine] ALTER TABLE customer DROP COLUMN email
INFO  [sqlalchemy.engine.base.Engine] {}
INFO  [sqlalchemy.engine.base.Engine] UPDATE alembic_version SET version_num='28af9800143f' WHERE alembic_version.version_num = '191a2d20b025'
INFO  [sqlalchemy.engine.base.Engine] {}
INFO  [sqlalchemy.engine.base.Engine] COMMIT

Don’t Generate Empty Migrations with Autogenerate

A common request is to have the alembic revision --autogenerate command not actually generate a revision file if no changes to the schema is detected. Using the EnvironmentContext.configure.process_revision_directives hook, this is straightforward; place a process_revision_directives hook in MigrationContext.configure() which removes the single MigrationScript directive if it is empty of any operations:

def run_migrations_online():

    # ...

    def process_revision_directives(context, revision, directives):
        if config.cmd_opts.autogenerate:
            script = directives[0]
            if script.upgrade_ops.is_empty():
                directives[:] = []


    # connectable = ...

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

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