Fix msgspec enum discriminator tag

Author: koxudaxi

src/datamodel_code_generator/parser/base.py

@@ -1794,7 +1794,7 @@ def get_enum_from_base(
                             has_one_literal = True
                             if isinstance(discriminator_model, msgspec_model.Struct):  # pragma: no cover
                                 discriminator_model.add_base_class_kwarg("tag_field", f"'{field_name}'")
-                                discriminator_model.add_base_class_kwarg("tag", discriminator_field.represented_default)
+                                discriminator_model.add_base_class_kwarg("tag", repr(expected_value))
                                 discriminator_field.extras["is_classvar"] = True
                             # Found the discriminator field, no need to keep looking
                             break
@@ -1811,6 +1811,20 @@ def get_enum_from_base(
                         if self.use_enum_values_in_discriminator:
                             enum_source = enum_source or get_enum_from_base()
 
+                        if (
+                            expected_value is not None
+                            and enum_source
+                            and isinstance(discriminator_model, msgspec_model.Struct)
+                        ):
+                            has_one_literal = True
+                            discriminator_model.add_base_class_kwarg("tag_field", f"'{field_name}'")
+                            discriminator_model.add_base_class_kwarg(
+                                "tag",
+                                repr(Parser._get_enum_discriminator_literal(enum_source, expected_value)),
+                            )
+                            discriminator_field.extras["is_classvar"] = True
+                            break
+
                         for field_data_type in discriminator_field.data_type.all_data_types:
                             if field_data_type.reference:  # pragma: no cover
                                 self.generation_store.detach_data_type_ref(field_data_type)

tests/data/expected/main/openapi/discriminator/enum_single_value_msgspec.py

@@ -0,0 +1,20 @@
+# generated by datamodel-codegen:
+#   filename:  discriminator_enum_single_value_msgspec.yaml
+#   timestamp: 2019-07-26T00:00:00+00:00
+
+from __future__ import annotations
+
+from typing import TypeAlias
+
+from msgspec import Struct
+
+
+class StartedEvent(Struct, tag_field='type', tag='started'):
+    pass
+
+
+class StoppedEvent(Struct, tag_field='type', tag='stopped'):
+    pass
+
+
+Event: TypeAlias = StartedEvent | StoppedEvent

tests/data/openapi/discriminator_enum_single_value_msgspec.yaml

@@ -0,0 +1,31 @@
+openapi: 3.1.0
+info:
+  title: Repro
+  version: 0.0.1
+paths: {}
+components:
+  schemas:
+    Event:
+      discriminator:
+        propertyName: type
+      oneOf:
+        - $ref: '#/components/schemas/StartedEvent'
+        - $ref: '#/components/schemas/StoppedEvent'
+    StartedEvent:
+      type: object
+      properties:
+        type:
+          type: string
+          enum:
+            - started
+      required:
+        - type
+    StoppedEvent:
+      type: object
+      properties:
+        type:
+          type: string
+          enum:
+            - stopped
+      required:
+        - type

tests/main/openapi/test_main_openapi.py

@@ -331,6 +331,28 @@ def test_main_openapi_discriminator_enum_single_value_anyof_use_enum(output_file
     )
 
 
+def test_main_openapi_discriminator_enum_single_value_msgspec(output_file: Path) -> None:
+    """Single-value enum discriminator is used as the msgspec tag."""
+    run_main_and_assert(
+        input_path=OPEN_API_DATA_PATH / "discriminator_enum_single_value_msgspec.yaml",
+        output_path=output_file,
+        input_file_type="openapi",
+        assert_func=assert_file_content,
+        expected_file=EXPECTED_OPENAPI_PATH / "discriminator" / "enum_single_value_msgspec.py",
+        extra_args=[
+            "--target-python-version",
+            "3.10",
+            "--output-model-type",
+            "msgspec.Struct",
+            "--enum-field-as-literal",
+            "all",
+            "--use-one-literal-as-default",
+            "--disable-warnings",
+        ],
+        force_exec_validation=True,
+    )
+
+
 def test_main_openapi_discriminator_with_properties(output_file: Path) -> None:
     """Test OpenAPI generation with discriminator properties."""
     run_main_and_assert(