Typing issue with `ForeignKeyNullableRelation`

[Pirulax]

Describe the bug
Using mypy I get typing errors:

Incompatible types in assignment (expression has type "User | None", variable has type "ForeignKeyFieldInstance[Any] | None")
Cannot assign to attribute "solved_by" for class "Ticket"
  Type "User | None" is not assignable to type "ForeignKeyNullableRelation[User]"
    Type "User" is not assignable to type "ForeignKeyNullableRelation[User]"
      "User" is not assignable to "ForeignKeyFieldInstance[User]"
      "User" is not assignable to "None"
(parameter) solved_by: User | None

To Reproduce
I have (pretty much) the following code:

from tortoise.models import Model

class User(Model):
    id = fields.IntField(unique=True, pk=True)

class Ticket(Model):
    id = fields.IntField(unique=True, pk=True)

    solved_by : fields.ForeignKeyNullableRelation[User] = fields.ForeignKeyField("User", "solved_tickets", null=True)

And the following is causing the issue:

async def set_solved_by(self, solved_by: User | None): # self here is a wrapper class with a state, `model` is an instance of `Ticket`
    self.model.solved_by = solved_by 

Expected behavior
To not get typing errors

Additional context
I'm not quite sure if I'm using the correct typing for the field, but I assume I do?

[waketzheng]

1. If User/Ticket is a table, they should be inherit from tortoise.models.Model
2. If you are using the latest version of tortoise-orm, (unique=True,pk=True) can be (primary_key=True)

Example::

from __future__ import annotations

from tortoise import Model, fields


class User(Model):
    id = fields.IntField(primary_key=True)
    name = fields.CharField(32, default="")

    solved_tickets: fields.ReverseRelation[Ticket]

    async def solve_it(self, ticket: Ticket) -> None:
        await ticket.set_solved_by(self)


class Ticket(Model):
    id = fields.IntField(primary_key=True)
    name = fields.CharField(32, default="")

    solved_by: fields.ForeignKeyNullableRelation[User] = fields.ForeignKeyField(
        "models.User", related_name="solved_tickets", null=True
    )

    async def set_solved_by(self, who: User | None) -> None:
        self.solved_by = who
        await self.save()


async def set_solved_by(ticket: Ticket, solved_by: User | None) -> None:
    # ticket = await Ticket.all().prefetch_related('solved_by').first()
    current_solved_by = ticket.solved_by
    if current_solved_by is not None and not isinstance(current_solved_by, User):
        # ticket = await Ticket.first()
        current_solved_by = await ticket.solved_by
    if current_solved_by is None:
        if solved_by is None:
            print("Nothing to do")
        else:
            await solved_by.solve_it(ticket)
            count = await solved_by.solved_tickets.all().count()
            if count == 1:
                print(f"This is the first one that solved by {solved_by}")
            else:
                print(f"There are {count} tickets that solved by {solved_by}")
    elif solved_by is None:
        await ticket.set_solved_by(None)
        print(f"Change solved_by of {ticket.id} from {current_solved_by=} to None")
    elif current_solved_by.id != solved_by.id:
        await ticket.set_solved_by(solved_by)
        print(f"Updated {ticket} {current_solved_by=} --> {solved_by=}")
    else:
        print("Already set, no need to update")

[Pirulax]

Yes, I'm sorry, I was in a hurry and yeah, the example was incomplete, I've updated it now.
But the issue stands nevertheless.