MappedAnnotationError with generic types using forward references

[zshuzh]

Hi!

I'm trying to use a TYPE_CHECKING block to resolve circular type imports, as described in this discussion. However, my use case is a bit more complex as it involves a generic type, and I can't seem to avoid a MappedAnnotationError error.

Minimal reproduction

# custom_id.py

# imports...

class WithTypePrefix(Protocol):
    @staticmethod
    def type_prefix() -> str: ...

class CustomID[T: WithTypePrefix](str): ...

# foo.py

# imports...

from .bar import Bar

class Foo(BaseEntity):
    __tablename__ = "foo"
    
    @staticmethod
    @override
    def type_prefix() -> TypePrefix:
        return TypePrefix("foo")

    async def contrived_function(self) -> None:
		# this function uses Bar

# bar.py

# imports...

if TYPE_CHECKING:
    from .foo import Foo

class Bar(BaseEntity):
    __tablename__ = "bar"
    
    @staticmethod
    @override
    def type_prefix() -> TypePrefix:
        return TypePrefix("bar")

    # This fails with MappedAnnotationError
    foo_id: Mapped[CustomID["Foo"]] = mapped_column(ForeignKey("foo.id"))

I get this error when I try to generate the migration with alembic —

MappedAnnotationError: Could not resolve all types within mapped annotation: "sqlalchemy.orm.base.Mapped[CustomID[ForwardRef('Foo')]]". Ensure all types are written correctly and are imported within the module in use.

---

The only thing that works is using Any instead of a forward reference, but this loses type safety so I'd like to avoid that if possible. Is there a known way to avoid MappedAnnotationError in this kind of generic, circular import situation?

[zzzeek]

I think you need to put a custom type like that in type_annotation_map, but im not sure if we're able to resolve a generic type like that even from the type map. But also I dont really understand what that whole thing is going to accomplish unless you are using a custom TypeDecorator as the SQL type? If you map to a plain string you're going to get str back which will not have a type_prefix() method.

i know you called this a "minimal reproduction" but it's still not clear what you actually mean to do.