Alembic: How to migrate custom type in a model?

Question

My User model is

class User(UserMixin, db.Model):
    __tablename__ = 'users'
    # noinspection PyShadowingBuiltins
    uuid = Column('uuid', GUID(), default=uuid.uuid4, primary_key=True,
                  unique=True)
    email = Column('email', String, nullable=False, unique=True)
    _password = Column('password', String, nullable=False)
    created_on = Column('created_on', sa.types.DateTime(timezone=True),
                        default=datetime.utcnow())
    last_login = Column('last_login', sa.types.DateTime(timezone=True),
                        onupdate=datetime.utcnow())

where GUID is a custom type as described in sqlalchemy docs (Exactly same)

Now when I run

alembic revision --autogenerate -m "Added initial table"

I get my upgrade() as

def upgrade():
    ### commands auto generated by Alembic - please adjust! ###
    op.create_table('users',
    sa.Column('uuid', sa.GUID(), nullable=False),
    sa.Column('email', sa.String(), nullable=False),
    sa.Column('password', sa.String(), nullable=False),
    sa.Column('created_on', sa.DateTime(timezone=True), nullable=True),
    sa.Column('last_login', sa.DateTime(timezone=True), nullable=True),
    sa.PrimaryKeyConstraint('uuid'),
    sa.UniqueConstraint('email'),
    sa.UniqueConstraint('uuid')
    )
    ### end Alembic commands ###

but during applying upgrade -> alembic upgrade head, I see

File "alembic/versions/49cc74d0da9d_added_initial_table.py", line 20, in upgrade
    sa.Column('uuid', sa.GUID(), nullable=False),
AttributeError: 'module' object has no attribute 'GUID'

How can I make it work with GUID/custom type here?

Answer 1

Short answer (using sqlalchemy version 1.4.25):

From the documentation:

For user-defined types, that is, any custom type that is not within the sqlalchemy. module namespace, by default Alembic will use the value of __module__ for the custom type:

Column("my_column", myapp.models.utils.types.MyCustomType())

The imports for the above type again must be made present within the migration, either manually, or by adding it to script.py.mako.

So, import your my_module in script.py.mako (and you probably need to define your custom type in a file other than models.py):

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

Long answer:

I have my custom type BinaryUUID defined in uuid_type_mysql.py and I'm importing it in models.py and using it there:

models.py:

...
from .uuid_type_mysql import BinaryUUID
...

After generating the migrations with flask db migrate I would get this on the migration file:

...
sa.Column('public_id', my_module.uuid_type_mysql.BinaryUUID(length=16), nullable=False),
...

And the problem is that the migration does not know my_module because it is not imported.

After adding import my_module to script.py.mako as suggested in the documentation, now the module is imported in the migration file:

...
from alembic import op
import sqlalchemy as sa
import my_module
...

After this, everything work fine for me with the bonus that it is not needed to manually edit the generated migration.

Answer 2

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!