Fun with SQLAlchemy Mapped forward refs

SQLAlchemy has nifty declarative mapping type support with the Mapped typing descriptor. If you are in the habit of typing your Python, this is a welcome tool that reduces boilerplate while still feeling quite natural:

class Banana(SqlalchemyBase):
	is_squishy:Mapped[Optional[bool]] = mapped_column(server_default=text("TRUE"))
	picked_date:Mapped[datetime]
	belongs_to: Mapped["User"] = relationship("User", lazy="selectin", back_references="bananas")

But interestingly, Mapped does not support forward refs outside the ORM. so picked_date:Mapped["datetime"] will error with an ArgumentError if datetime was not otherwise imported in the module. However, our forward ref to "User" will be fine as long as you’ve imported User via TYPE_CHECKING.

So in conclusion, this works:

...
from typing import TYPE_CHECKING

if TYPE_CHECKING:
	from . import User # avoids circular refs

class Banana(SqlalchemyBase):
	is_squishy:Mapped[Optional[bool]] = mapped_column(server_default=text("TRUE"))
	picked_date:Mapped[datetime]
	belongs_to: Mapped["User"] = relationship("User", lazy="selectin", back_references="bananas")

but if you try to forward ref anything not inheriting from SqlalchemyBase, 🙀!