MappedAsDataclass applies @dataclasses.dataclass unconditionally

When using the :class:`.MappedAsDataclass` superclass, all classes within
the hierarchy that are subclasses of this class will now be run through the
``@dataclasses.dataclass`` function whether or not they are actually
mapped, so that non-ORM fields declared on non-mapped classes within the
hierarchy will be used when mapped subclasses are turned into dataclasses.
This behavior applies both to intermediary classes mapped with
``__abstract__ = True`` as well as to the user-defined declarative base
itself, assuming :class:`.MappedAsDataclass` is present as a superclass for
these classes.

This allows non-mapped attributes such as ``InitVar`` declarations on
superclasses to be used, without the need to run the
``@dataclasses.dataclass`` decorator explicitly on each non-mapped class.
The new behavior is considered as correct as this is what the :pep:`681`
implementation expects when using a superclass to indicate dataclass
behavior.

Fixes: #9179
Change-Id: Ia01fa9806a27f7c1121bf7eaddf2847cf6dc5313

2 files changed, 22 insertions, 4 deletions

diff --git a/lib/sqlalchemy/orm/decl_api.py b/lib/sqlalchemy/orm/decl_api.py
index ecd81c7ed..c0bc37008 100644
--- a/lib/sqlalchemy/orm/decl_api.py
+++ b/lib/sqlalchemy/orm/decl_api.py
@@ -45,6 +45,7 @@ from ._orm_constructors import relationship
 from ._orm_constructors import synonym
 from .attributes import InstrumentedAttribute
 from .base import _inspect_mapped_class
+from .base import _is_mapped_class
 from .base import Mapped
 from .decl_base import _add_attribute
 from .decl_base import _as_declarative
@@ -596,20 +597,29 @@ class MappedAsDataclass(metaclass=DCTransformDeclarative):
             "kw_only": kw_only,
         }
 
+        current_transforms: _DataclassArguments
+
         if hasattr(cls, "_sa_apply_dc_transforms"):
             current = cls._sa_apply_dc_transforms  # type: ignore[attr-defined]
 
             _ClassScanMapperConfig._assert_dc_arguments(current)
 
-            cls._sa_apply_dc_transforms = {
+            cls._sa_apply_dc_transforms = current_transforms = {  # type: ignore  # noqa: E501
                 k: current.get(k, _NoArg.NO_ARG) if v is _NoArg.NO_ARG else v
                 for k, v in apply_dc_transforms.items()
             }
         else:
-            cls._sa_apply_dc_transforms = apply_dc_transforms
+            cls._sa_apply_dc_transforms = (
+                current_transforms
+            ) = apply_dc_transforms
 
         super().__init_subclass__()
 
+        if not _is_mapped_class(cls):
+            _ClassScanMapperConfig._apply_dataclasses_to_any_class(
+                current_transforms, cls
+            )
+
 
 class DeclarativeBase(
     inspection.Inspectable[InstanceState[Any]],
diff --git a/lib/sqlalchemy/orm/decl_base.py b/lib/sqlalchemy/orm/decl_base.py
index 9e8b02359..0462a8945 100644
--- a/lib/sqlalchemy/orm/decl_base.py
+++ b/lib/sqlalchemy/orm/decl_base.py
@@ -1078,10 +1078,18 @@ class _ClassScanMapperConfig(_MapperConfig):
 
         self.cls.__annotations__ = annotations
 
-        self._assert_dc_arguments(dataclass_setup_arguments)
+        self._apply_dataclasses_to_any_class(
+            dataclass_setup_arguments, self.cls
+        )
+
+    @classmethod
+    def _apply_dataclasses_to_any_class(
+        cls, dataclass_setup_arguments: _DataclassArguments, klass: Type[_O]
+    ) -> None:
+        cls._assert_dc_arguments(dataclass_setup_arguments)
 
         dataclasses.dataclass(
-            self.cls,
+            klass,
             **{
                 k: v
                 for k, v in dataclass_setup_arguments.items()