How to prevent alembic revision –autogenerate from making revision file if it has not detected any changes?

I have project where I’m using SQLAlchemy for models and I’m trying to integrate Alembic for making migrations. Everything works as expected when I change models and Alembic sees that models have changed -> it creates good migration file with command: alembic revision --autogenerate -m "model changed"

But when I have NOT changed anything in models and I use the same command:

alembic revision --autogenerate -m "should be no migration"

revision gives me ’empty’ revision file like this:

"""next

Revision ID: d06d2a8fed5d
Revises: 4461d5328f57
Create Date: 2021-12-02 18:09:42.208607

"""
from alembic import op
import sqlalchemy as sa


# revision identifiers, used by Alembic.
revision = 'd06d2a8fed5d'
down_revision = '4461d5328f57'
branch_labels = None
depends_on = None


def upgrade():
    # ### commands auto generated by Alembic - please adjust! ###
    pass
    # ### end Alembic commands ###


def downgrade():
    # ### commands auto generated by Alembic - please adjust! ###
    pass
    # ### end Alembic commands ###

What is purpose of this file? Could I prevent creation of this ’empty file’ when alembic revision –autogenerate will not see any changes? To compare when I use Django and it’s internal migration when I type command:

python manage.py makemigrations

I get output something like:

No changes detected

and there is not migration file created. Is there a way to do the same with Alembic revision? Or is there other command that could check if there were changes in models and if there were then I could simply run alembic revision and upgrade?

Answer

Accepted answer does not answer the question. The correct answer is: Yes, you can call alembic revision --autogenerate and be sure that only if there are changes a revision file would be generated:

As per alembic’s documentation

Implemented in Flask-Migrate (exactly in this file), it’s just a change to env.py to account for the needed feature, namely to not autogenerate a revision if there are no changes to the models.

You would still run alembic revision --autogenerate -m "should be no migration" but the change you would make to the env.py is, in short:

    def run_migrations_online():
        # almost identical to Flask-Migrate (Thanks miguel!)    
        # this callback is used to prevent an auto-migration from being generated
        # when there are no changes to the schema
    
        def process_revision_directives(context, revision, directives):
            if config.cmd_opts.autogenerate:
                script = directives[0]
                if script.upgrade_ops.is_empty():
                    directives[:] = []
                    print('No changes in schema detected.')
    
        connectable = engine_from_config(
            config.get_section(config.config_ini_section),
            prefix="sqlalchemy.",
            poolclass=pool.NullPool,
        )
    
        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() 

Now, you can easily call alembic revision --autogenerate without risking the creation of a new empty revision.