Alembic: How to migrate custom type in a model?

I had a similar problem and solved it like follows:

Let's assume you have the following module my_guid, containing (from the page you already cited, with minor naming modifications):

import uuid as uuid_package
from sqlalchemy.dialects.postgresql import UUID as PG_UUID
from sqlalchemy import TypeDecorator, CHAR

class GUID(TypeDecorator):
    impl = CHAR

    def load_dialect_impl(self, dialect):
        if dialect.name == 'postgresql':
            return dialect.type_descriptor(PG_UUID())
        else:
            return dialect.type_descriptor(CHAR(32))

    def process_bind_param(self, value, dialect):
        if value is None:
            return value
        elif dialect.name == 'postgresql':
            return str(value)
        else:
            if not isinstance(value, uuid_package.UUID):
                return "%.32x" % uuid_package.UUID(value)
            else:
                # hexstring
                return "%.32x" % value

    def process_result_value(self, value, dialect):
        if value is None:
            return value
        else:
            return uuid_package.UUID(value)

If you use this GUID in your models, you just need to add three lines at alembic/env.py:

from my_guid import GUID
import sqlalchemy as sa
sa.GUID = GUID

That worked for me. Hope that helps!

You can replace sa.GUID() with either sa.CHAR(32) or UUID() (after adding the import line from sqlalchemy.dialects.postgresql import UUID) depending on the dialect.

Replacing it with GUID() (after adding the import line from your.models.custom_types import GUID) will work also, but then the upgrade script is tied to your model code, which may not be a good thing.