Migrate alembic from monodb to multidb?

[Diego-Hernandez-Moodys]

We've already initialized our project using a template for a single database.

Is there any easy way for us to

1. Migrate our alembic.ini and migrations/env.py to use the multidb template
2. Retain the modification versions but point to a single database?

[CaselIT]

Hi,

I would init alembic using the multi-db template into a scratch directory and compare it with what you have, and apply the required changes to your current setup. They should not be too many

[Diego-Hernandez-Moodys]

I've made some progress and now I'm working on migrating the versions.
Is the only thing differentiating different databases in the multi-db is that the downgrade and update functions refer to the database explicitly?

e.g., old way

"""Add official name

Revision ID: e34c2e65c6df
Revises: ef2f18a8fd24
Create Date: 2024-01-30 10:00:14.273803

"""
import alembic
import sqlalchemy

# revision identifiers, used by Alembic.
revision = 'e34c2e65c6df'
down_revision = 'ef2f18a8fd24'
branch_labels = None
depends_on = None

def upgrade():
    alembic.op.add_column(
        "org",
        sqlalchemy.Column("vendor_name", sqlalchemy.VARCHAR(256), nullable=True, insert_after="name") 
    )

def downgrade():
    alembic.op.drop_column("org", "vendor_name")

Now I just change the downgrade and upgrade functions to reflect the engine?

"""Add official name

Revision ID: e34c2e65c6df
Revises: ef2f18a8fd24
Create Date: 2024-01-30 10:00:14.273803

"""
import alembic
import sqlalchemy

# revision identifiers, used by Alembic.
revision = 'e34c2e65c6df'
down_revision = 'ef2f18a8fd24'
branch_labels = None
depends_on = None

def upgrade_engine1():
    alembic.op.add_column(
        "org",
        sqlalchemy.Column("vendor_name", sqlalchemy.VARCHAR(256), nullable=True, insert_after="name") 
    )

def downgrade_engine1():
    alembic.op.drop_column("org", "vendor_name")

Do I still need these functions present?

def upgrade(engine_name):
    globals()["upgrade_%s" % engine_name]()


def downgrade(engine_name):
    globals()["downgrade_%s" % engine_name]()

[CaselIT]

these are created by the mako template used by alembic. The multidb one uses that:

alembic/alembic/templates/multidb/script.py.mako

Lines 24 to 29 in 8829bd6

	def upgrade(engine_name: str) -> None:
	globals()["upgrade_%s" % engine_name]()
	def downgrade(engine_name: str) -> None:
	globals()["downgrade_%s" % engine_name]()

[Diego-Hernandez-Moodys]

Yes I saw that in the readme, but now I'm porting existing versions/*.py to the new format. What's minimally required?

[CaselIT]

in your case I think just renaming existing upgrade and downgrade to have the engine name you wanted, like you already shown, then adding to all migration the "standard" upgrade/downgrade from the multi template

def upgrade(engine_name: str) -> None: 
     globals()["upgrade_%s" % engine_name]() 
  
  
def downgrade(engine_name: str) -> None: 
     globals()["downgrade_%s" % engine_name]() 

[Diego-Hernandez-Moodys]

That worked! Thank you.
One final note is the alembic_version in the database for the additional engine2 needs to have a row that points to head revision id, otherwise you'll get the

ERROR [alembic.util.messaging] Destination head is not a valid downgrade target from current head(s)
  FAILED: Destination head is not a valid downgrade target from current head(s)

error.