Fix allOf merging logic to handle single $ref inheritance with property overrides (#2647)

Author: koxudaxi

src/datamodel_code_generator/parser/jsonschema.py

@@ -1365,7 +1365,15 @@ def _schema_signature(self, prop_schema: JsonSchemaObject | bool) -> str | bool:
         return json.dumps(prop_schema.dict(exclude_unset=True, by_alias=True), sort_keys=True, default=repr)
 
     def _merge_all_of_object(self, obj: JsonSchemaObject) -> JsonSchemaObject | None:
-        """Merge allOf items when they share object properties to avoid duplicate models."""
+        """Merge allOf items when they share object properties to avoid duplicate models.
+
+        Skip merging when there is exactly one $ref (inheritance with property overrides).
+        Continue merging when multiple $refs share properties to avoid duplicate fields.
+        """
+        ref_count = sum(1 for item in obj.allOf if item.ref)
+        if ref_count == 1:
+            return None
+
         resolved_items: list[JsonSchemaObject] = []
         property_signatures: dict[str, set[str | bool]] = {}
         for item in obj.allOf:

tests/data/expected/main/jsonschema/all_of_ref_with_property_override.py

@@ -0,0 +1,20 @@
+# generated by datamodel-codegen:
+#   filename:  all_of_ref_with_property_override.json
+#   timestamp: 2019-07-26T00:00:00+00:00
+
+from __future__ import annotations
+
+from typing import Any, Optional
+
+from pydantic import BaseModel, constr
+
+
+class Thing(BaseModel):
+    type: str
+    name: constr(min_length=1)
+
+
+class Person(Thing):
+    type: Optional[Any] = 'playground:Person'
+    name: Optional[Any] = None
+    age: Optional[int] = None

tests/data/jsonschema/all_of_ref_with_property_override.json

@@ -0,0 +1,24 @@
+{
+  "$schema": "https://json-schema.org/draft/2020-12/schema",
+  "$id": "https://example.com/person.schema.json",
+  "title": "Person",
+  "$defs": {
+    "Thing": {
+      "type": "object",
+      "properties": {
+        "type": { "type": "string" },
+        "name": { "type": "string", "minLength": 1 }
+      },
+      "required": ["type", "name"]
+    }
+  },
+  "type": "object",
+  "allOf": [
+    { "$ref": "#/$defs/Thing" }
+  ],
+  "properties": {
+    "type": { "default": "playground:Person" },
+    "name": { "minLength": 2 },
+    "age": { "type": "integer" }
+  }
+}

tests/main/jsonschema/test_main_jsonschema.py

@@ -936,6 +936,18 @@ def test_main_all_of_merge_boolean_property(output_file: Path) -> None:
         )
 
 
+def test_main_all_of_ref_with_property_override(output_file: Path) -> None:
+    """Test allOf with $ref preserves inheritance when properties are overridden."""
+    with chdir(JSON_SCHEMA_DATA_PATH):
+        run_main_and_assert(
+            input_path=Path("all_of_ref_with_property_override.json"),
+            output_path=output_file,
+            input_file_type="jsonschema",
+            assert_func=assert_file_content,
+            expected_file="all_of_ref_with_property_override.py",
+        )
+
+
 @pytest.mark.skipif(
     black.__version__.split(".")[0] >= "24",
     reason="Installed black doesn't support the old style",