fix: preserve Meta() constraints on optional msgspec Struct fields

litestar/plugins/core/_msgspec.py

@@ -37,8 +37,16 @@ def get_typed_init(self, type_: Any) -> tuple[Signature, dict[str, Any]]:
 def kwarg_definition_from_field(field: msgspec.inspect.Field) -> tuple[ParameterKwarg | None, dict[str, Any]]:
     extra: dict[str, Any] = {}
     kwargs: dict[str, Any] = {}
+    meta: msgspec.inspect.Metadata | None = None
     if isinstance(field.type, msgspec.inspect.Metadata):
         meta = field.type
+    elif isinstance(field.type, msgspec.inspect.UnionType):
+        meta = next(
+            (member for member in field.type.types if isinstance(member, msgspec.inspect.Metadata)),
+            None,
+        )
+
+    if meta is not None:
         field_type = meta.type
         if extra_json_schema := meta.extra_json_schema:
             kwargs["title"] = extra_json_schema.get("title")

tests/unit/test_openapi/test_schema.py

@@ -745,3 +745,32 @@ def handler(query_param: Annotated[annotation, Parameter(description="foo")]) ->
     assert param.schema.type is OpenAPIType.INTEGER  # type: ignore[union-attr]
     # ensure other attributes than the plain type are carried over correctly
     assert param.description == "foo"
+
+
+def test_optional_field_preserves_meta_constraints() -> None:
+    """Regression test for https://github.com/litestar-org/litestar/issues/4650.
+
+    ``kwarg_definition_from_field`` must extract ``Meta()`` constraints from
+    optional fields (where the inspected type is a ``UnionType`` wrapping
+    ``Metadata``), so that constraints like ``ge``, ``le``, and ``examples``
+    are not silently dropped from the generated OpenAPI schema.
+    """
+
+    class Model(Struct, kw_only=True):
+        required_field: Annotated[int, msgspec.Meta(ge=1, examples=[42])]
+        optional_field: Annotated[int, msgspec.Meta(ge=1, examples=[42])] | None = None
+
+    properties = get_schema_for_field_definition(
+        FieldDefinition.from_kwarg(name="Model", annotation=Model)
+    ).properties
+
+    assert properties is not None
+
+    # required_field should have constraints
+    assert properties["required_field"].minimum == 1.0
+    assert properties["required_field"].examples == [42]
+
+    # optional_field should be oneOf with constraints preserved on the schema
+    assert properties["optional_field"].one_of is not None
+    assert properties["optional_field"].minimum == 1
+    assert properties["optional_field"].examples == [42]