Alembic support for multiple Postgres schemas

Question

How can I use Alembic\'s --autogenerate to migrate multiple Postgres schemas that are not hard-coded in the SQL Alchemy model? (mirror question of SQLA

Answer 1

from issue 409, the application to tenant-specific schemas of upgrade/downgrade operations can most easily be done using translated schema names, which is also how you would normally be doing the main application as well for multi-tenant.

Go into env.py:

def run_migrations_online():

    connectable = engine_from_config(
        config.get_section(config.config_ini_section),
        prefix='sqlalchemy.',
        poolclass=pool.NullPool)

    with connectable.connect() as connection:
        for tenant_schema_name in all_my_tenant_names:
             conn = connection.execution_options(schema_translate_map={None: tenant_schema_name}

            logger.info("Migrating tenant schema %s" % tenant_schema_name)
            context.configure(
                connection=conn,
                target_metadata=target_metadata
            )

            # to do each tenant in its own transaction.
            # move this up to do all tenants in one giant transaction
            with context.begin_transaction():
                context.run_migrations()

Above will translate the "None" schema name into the given tenant name. If the application shares tenant-based schemas with a default schema that has global tables, then you'd be using some token like "tenant_schema" as the symbol:

for tenant_schema_name in all_my_tenant_names:
     conn = connection.execution_options(schema_translate_map={"tenant_schema": tenant_schema_name}

and in migration files refer to "tenant_schema" where the actual tenant-specific schema name goes:

def upgrade():
    op.alter_column("some_table", "some_column", , schema="tenant_schema")

For the "autogenerate" case, the solution @nick-retallack provides has some more of the pieces you would use on that end, namely the use of include_schemas so that autogenerate looks only at a "specimen" schema that represents the latest version of the tenant-specific schema.

In order to set up env.py to use the right system for the right command, the behaviors can be controlled using user-defined options with migration_context.get_x_argument().