Bug report

What's wrong

The typing of a ManyToMany relation's add() method incorrectly assumes that model ids are int, but they can be other types, such as UUID. Attempting to call add(model_id) causes a mypy error if the id is not an int:

Argument 1 has incompatible type "UUID"; expected "Union[FooModel, int]"

How is that should be

The add() method should accept the related model, and whatever the type of the related model's id is.