From 446d8bebf69cc49136ac9bd886acc2ed5641483d Mon Sep 17 00:00:00 2001
From: Koudai Aono <koxudaxi@gmail.com>
Date: Sat, 20 Dec 2025 16:18:45 +0000
Subject: [PATCH] fix: skip non-model types in __change_field_name

---
 src/datamodel_code_generator/parser/base.py   |  2 +
 .../main/graphql/union_snake_case_field.py    | 63 +++++++++++++++++++
 tests/main/graphql/test_main_graphql.py       | 12 ++++
 3 files changed, 77 insertions(+)
 create mode 100644 tests/data/expected/main/graphql/union_snake_case_field.py

diff --git a/src/datamodel_code_generator/parser/base.py b/src/datamodel_code_generator/parser/base.py
index 747929fa0..600a2bc49 100644
--- a/src/datamodel_code_generator/parser/base.py
+++ b/src/datamodel_code_generator/parser/base.py
@@ -1805,6 +1805,8 @@ def __change_field_name(
         for model in models:
             if "Enum" in model.base_class:
                 continue
+            if not model.BASE_CLASS:
+                continue
 
             for field in model.fields:
                 filed_name = field.name
diff --git a/tests/data/expected/main/graphql/union_snake_case_field.py b/tests/data/expected/main/graphql/union_snake_case_field.py
new file mode 100644
index 000000000..71c9a4e39
--- /dev/null
+++ b/tests/data/expected/main/graphql/union_snake_case_field.py
@@ -0,0 +1,63 @@
+# generated by datamodel-codegen:
+#   filename:  union.graphql
+#   timestamp: 2019-07-26T00:00:00+00:00
+
+from __future__ import annotations
+
+from typing import Literal, Union
+
+from pydantic import BaseModel, Field
+from typing_extensions import TypeAliasType
+
+Boolean = TypeAliasType("Boolean", bool)
+"""
+The `Boolean` scalar type represents `true` or `false`.
+"""
+
+
+ID = TypeAliasType("ID", str)
+"""
+The `ID` scalar type represents a unique identifier, often used to refetch an object or as key for a cache. The ID type appears in a JSON response as a String; however, it is not intended to be human-readable. When expected as an input type, any string (such as `"4"`) or integer (such as `4`) input value will be accepted as an ID.
+"""
+
+
+Int = TypeAliasType("Int", int)
+"""
+The `Int` scalar type represents non-fractional signed whole numeric values. Int can represent values between -(2^31) and 2^31 - 1.
+"""
+
+
+String = TypeAliasType("String", str)
+"""
+The `String` scalar type represents textual data, represented as UTF-8 character sequences. The String type is most often used by GraphQL to represent free-form human-readable text.
+"""
+
+
+class IResource(BaseModel):
+    id: ID
+    typename__: Literal['IResource'] | None = Field('IResource', alias='__typename')
+
+
+class Car(IResource):
+    id: ID
+    passenger_capacity: Int = Field(..., alias='passengerCapacity')
+    typename__: Literal['Car'] | None = Field('Car', alias='__typename')
+
+
+class Employee(IResource):
+    first_name: String | None = Field(None, alias='firstName')
+    id: ID
+    last_name: String | None = Field(None, alias='lastName')
+    typename__: Literal['Employee'] | None = Field('Employee', alias='__typename')
+
+
+Resource = TypeAliasType(
+    "Resource",
+    Union[
+        'Car',
+        'Employee',
+    ],
+)
+
+
+TechnicalResource = TypeAliasType("TechnicalResource", Car)
diff --git a/tests/main/graphql/test_main_graphql.py b/tests/main/graphql/test_main_graphql.py
index ed6ef850c..b4c25c7ca 100644
--- a/tests/main/graphql/test_main_graphql.py
+++ b/tests/main/graphql/test_main_graphql.py
@@ -601,3 +601,15 @@ def test_main_graphql_dataclass_frozen_keyword_only(output_file: Path) -> None:
             "3.10",
         ],
     )
+
+
+def test_main_graphql_union_snake_case_field(output_file: Path) -> None:
+    """Test that union type references are not converted to snake_case."""
+    run_main_and_assert(
+        input_path=GRAPHQL_DATA_PATH / "union.graphql",
+        output_path=output_file,
+        input_file_type="graphql",
+        assert_func=assert_file_content,
+        expected_file="union_snake_case_field.py",
+        extra_args=["--snake-case-field", "--output-model-type", "pydantic_v2.BaseModel"],
+    )