Guard against wrong dataclass mapping

Ensure that the decorator style @registry.mapped_as_dataclass and
MappedAsDataclass are not mixed.

Fixes: #9211
Change-Id: I5cd94cae862122e4f627d0d051495b3789cf6de5

3 files changed, 38 insertions, 0 deletions

diff --git a/doc/build/changelog/unreleased_20/9211.rst b/doc/build/changelog/unreleased_20/9211.rst
new file mode 100644
index 000000000..f15a64b05
--- /dev/null
+++ b/doc/build/changelog/unreleased_20/9211.rst
@@ -0,0 +1,10 @@
+.. change::
+    :tags: bug, orm
+    :tickets: 9211
+
+    An explicit error is raised if a mapping attempts to mix the use of
+    :class:`_orm.MappedAsDataclass` with
+    :meth:`_orm.registry.mapped_as_dataclass` within the same class hierarchy,
+    as this produces issues with the dataclass function being applied at the
+    wrong time to the mapped class, leading to errors during the mapping
+    process.
diff --git a/lib/sqlalchemy/orm/decl_base.py b/lib/sqlalchemy/orm/decl_base.py
index a858f12cb..8ce9d59d0 100644
--- a/lib/sqlalchemy/orm/decl_base.py
+++ b/lib/sqlalchemy/orm/decl_base.py
@@ -1036,6 +1036,17 @@ class _ClassScanMapperConfig(_MapperConfig):
         if not dataclass_setup_arguments:
             return
 
+        # can't use is_dataclass since it uses hasattr
+        if "__dataclass_fields__" in self.cls.__dict__:
+            raise exc.InvalidRequestError(
+                f"Class {self.cls} 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'"
+            )
+
         manager = instrumentation.manager_of_class(self.cls)
         assert manager is not None
 
diff --git a/test/orm/declarative/test_dc_transforms.py b/test/orm/declarative/test_dc_transforms.py
index 63450f4a1..302587b9a 100644
--- a/test/orm/declarative/test_dc_transforms.py
+++ b/test/orm/declarative/test_dc_transforms.py
@@ -629,6 +629,23 @@ class DCTransformsTest(AssertsCompiledSQL, fixtures.TestBase):
         b1 = Bar(mixin_value=5)
         eq_(b1.bar_value, 78)
 
+    def test_mixing_MappedAsDataclass_with_decorator_raises(self, registry):
+        """test #9211"""
+
+        class Mixin(MappedAsDataclass):
+            id: Mapped[int] = mapped_column(primary_key=True, init=False)
+
+        with expect_raises_message(
+            exc.InvalidRequestError,
+            "Class .*Foo.* is already a dataclass; ensure that "
+            "base classes / decorator styles of establishing dataclasses "
+            "are not being mixed. ",
+        ):
+
+            @registry.mapped_as_dataclass
+            class Foo(Mixin):
+                bar_value: Mapped[float] = mapped_column(default=78)
+
 
 class RelationshipDefaultFactoryTest(fixtures.TestBase):
     def test_list(self, dc_decl_base: Type[MappedAsDataclass]):