SQLAlchemy 2.1 `MappedAsDataclass` compatibility

[krzysdz]

A change to SQLAlchemy 2.1 dataclass handling made it incompatible with Flask-SQLAlchemy. This simple code snippet is enough to reproduce the problem:

from flask_sqlalchemy import SQLAlchemy
from sqlalchemy.orm import DeclarativeBase, MappedAsDataclass

class BaseModel(DeclarativeBase, MappedAsDataclass):
    pass

db = SQLAlchemy(model_class=BaseModel)
# sqlalchemy.exc.InvalidRequestError: Class <class '__main__.FlaskSQLAlchemyBase'> is already a dataclass; ensure that base classes / decorator styles of establishing dataclasses are not being mixed. This can happen if a class that inherits from 'MappedAsDataclass', even indirectly, is been mapped with '@registry.mapped_as_dataclass'

Flask-SQLAlchemy uses types.new_class() to re-create user's model_class with added mixins. Because it also copies all attributes from the class namespace, the newly created class already has some attributes (specifically __dataclass_fields__) that MappedAsDataclass tries to add later when its __init_subclass__() is executed again. Since these attributes already exist, SQLAlchemy throws an error.

flask-sqlalchemy/src/flask_sqlalchemy/extension.py

Lines 544 to 555 in 168cb4b

	elif len(declarative_bases) == 1:
	body = dict(model_class.__dict__)
	body["__fsa__"] = self
	mixin_classes = [BindMixin, NameMixin, Model]
	if disable_autonaming:
	mixin_classes.remove(NameMixin)
	model = types.new_class(
	"FlaskSQLAlchemyBase",
	(*mixin_classes, *model_class.__bases__),
	{"metaclass": type(declarative_bases[0])},
	lambda ns: ns.update(body),
	)

I asked about it in the SQLAlchemy repository and it is not a bug in SQLAlchemy (sqlalchemy/sqlalchemy#13160).

Environment:

• Python version: 3.14.3
• Flask-SQLAlchemy version: 3.1.1
• SQLAlchemy version: 2.1.0b1

[davidism]

Supporting these SQLAlchemy features while keeping the legacy behaviors of Flask-SQLAlchemy has become untennable. You're welcome to submit a (human created) PR, but otherwise we suggest you use Flask-SQLAlchemy-Lite which provides the configuration and mangement without the custom behaviors.