Support sqlmodel_rebuild

The `@sqlmodel` decorator introduced by `fquery.sqlmodel` moves schema
definitions and types closer to the object model (references) instead of
foreign_keys.

Instead of writing:

```
class Hero(SQLModel, table=True):
    ...
    team_id: int | None = Field(default=None, foreign_key="team.id")
```

you could write:

```
@sqlmodel
class Hero:
    ...
    team: Team | None = foreign_key("team.id")
```

Everything works the same as before. Only the syntax has changed.
However, if `Team` also has a foreign key to `Hero`, then one of them
will have to use forward declaration so type checkers work.

In those cases we need a two pass solution where in the second pass,
with the full type definition available, we can generate the correct
code.

Refactor some of the existing code to introduce `sqlmodel_rebuild()`.

Author: adsharma

sqlmodel/main.py

@@ -585,9 +585,24 @@ def get_config(name: str) -> Any:
             setattr(new_cls, "__abstract__", True)  # noqa: B010
         return new_cls
 
-    # Override SQLAlchemy, allow both SQLAlchemy and plain Pydantic models
     def __init__(
         cls, classname: str, bases: Tuple[type, ...], dict_: Dict[str, Any], **kw: Any
+    ) -> None:
+        return cls.__do_init__(bases, dict_, kw)  # type: ignore
+
+    def sqlmodel_rebuild(cls) -> None:
+        reg = cls._sa_registry
+        # clear any exisiting mappers for the cls
+        managers = [m for m in reg._managers if m.class_ == cls]
+        for m in managers:
+            reg._dispose_manager_and_mapper(m)
+            del reg._managers[m]
+
+        return cls.__do_init__(cls.__bases__, cls.__dict__, {})  # type: ignore
+
+    # Override SQLAlchemy, allow both SQLAlchemy and plain Pydantic models
+    def __do_init__(
+        cls, classname: str, bases: Tuple[type, ...], dict_: Dict[str, Any], **kw: Any
     ) -> None:
         # Only one of the base classes (or the current one) should be a table model
         # this allows FastAPI cloning a SQLModel for the response_model without