Raise error when Pydantic dataclasses are used on models

docs/errors/usage_errors.md

@@ -1117,4 +1117,22 @@ except PydanticUserError as exc_info:
  assert exc_info.code == 'model-config-invalid-field-name'
 ```
 
+## `dataclass` is used on a `BaseModel` subclass {#dataclass-on-model}
+
+This error is raised when the Pydantic `dataclass` decorator is used on a class which is already
+a Pydantic model.
+
+```py
+from pydantic import BaseModel, PydanticUserError
+from pydantic.dataclasses import dataclass
+
+try:
+
+ @dataclass
+ class Model(BaseModel):
+ bar: str
+
+except PydanticUserError as exc_info:
+ assert exc_info.code == 'dataclass-on-model'
+
 {% endraw %}

pydantic/dataclasses.py

@@ -12,6 +12,7 @@
 from ._internal import _dataclasses as _pydantic_dataclasses
 from ._migration import getattr_migration
 from .config import ConfigDict
+from .errors import PydanticUserError
 from .fields import Field, FieldInfo, PrivateAttr
 
 if TYPE_CHECKING:
@@ -190,6 +191,14 @@ def create_dataclass(cls: type[Any]) -> type[PydanticDataclass]:
  Returns:
  A Pydantic dataclass.
  """
+ from ._internal._utils import is_model_class
+
+ if is_model_class(cls):
+ raise PydanticUserError(
+ f'Cannot create a Pydantic dataclass from {cls.__name__} as it is already a Pydantic model',
+ code='dataclass-on-model',
+ )
+
  original_cls = cls
 
  config_dict = config

pydantic/errors.py

@@ -62,6 +62,7 @@
  'dataclass-init-false-extra-allow',
  'clashing-init-and-init-var',
  'model-config-invalid-field-name',
+ 'dataclass-on-model',
 ]
 
 

tests/test_dataclasses.py

@@ -40,6 +40,16 @@
 from pydantic.json_schema import model_json_schema
 
 
+def test_cannot_create_dataclass_from_basemodel_subclass():
+ msg = 'Cannot create a Pydantic dataclass from SubModel as it is already a Pydantic model'
+
+ with pytest.raises(PydanticUserError, match=msg):
+
+ @pydantic.dataclasses.dataclass
+ class SubModel(BaseModel):
+ pass
+
+
 def test_simple():
  @pydantic.dataclasses.dataclass
  class MyDataclass: