fix(dto/msgspec_dto): Fix KeyError when ClassVar exists on msgspec dto struct (#4669)

Co-authored-by: аппаап <email@example.com>

Author: Peopl3s

docs/release-notes/changelog.rst

@@ -362,3 +362,9 @@
         Add a new attribute :attr:`~litestar.middleware.ASGIMiddleware.should_bypass_for_scope`;
         A callable which takes in a :class:`~litestar.types.Scope` and returns a boolean
         to indicate whether to bypass the middleware for the current request.
+
+    .. change:: Fix KeyError when ClassVar exists on msgspec Struct
+        :type: bugfix
+        :pr: 4665
+
+        Fix a bug in :class:`MsgspecDTO` where a KeyError was raised if a :class:`msgspec.Struct` contained a :class:`~typing.ClassVar`. ClassVars are now correctly skipped when generating field definitions.

litestar/dto/msgspec_dto.py

@@ -48,6 +48,8 @@ def generate_field_definitions(cls, model_type: type[Struct]) -> Generator[DTOFi
         property_fields = cls.get_property_fields(model_type)
 
         for key, field_definition in cls.get_model_type_hints(model_type).items():
+            if key not in inspect_fields:
+                continue
             kwarg_definition, extra = kwarg_definition_from_field(inspect_fields[key])
             field_definition = dataclasses.replace(field_definition, kwarg_definition=kwarg_definition)
             field_definition.extra.update(extra)

tests/unit/test_contrib/test_msgspec.py

@@ -2,7 +2,7 @@
 
 import itertools
 from dataclasses import replace
-from typing import TYPE_CHECKING, Annotated
+from typing import TYPE_CHECKING, Annotated, ClassVar
 from unittest.mock import ANY
 
 import pytest
@@ -216,3 +216,15 @@ def handler_3(data: Model3) -> None:
         "required": ["foo", "regular_field"],
         "title": "Model3",
     }
+
+
+def test_msgspec_dto_with_classvar() -> None:
+    class ModelWithClassVar(Struct):
+        regular_field: str
+        class_field: ClassVar[str] = "a string in the class"
+
+    field_defs = list(MsgspecDTO.generate_field_definitions(ModelWithClassVar))
+
+    # Only the regular field should be included, not the ClassVar
+    assert len(field_defs) == 1
+    assert field_defs[0].name == "regular_field"