Fix namespace subnamespace reference resolution in OpenAPI generation (#2580)

Author: koxudaxi

src/datamodel_code_generator/parser/base.py

@@ -815,7 +815,7 @@ def __change_from_import(
                 if from_ and import_ and alias != name:
                     data_type.alias = alias if data_type.reference.short_name == import_ else f"{alias}.{name}"
 
-                if init:
+                if init and not data_type.full_name.startswith(model.module_name + "."):
                     from_ = "." + from_
                 imports.append(
                     Import(

tests/data/expected/main/openapi/namespace_subns_ref/__init__.py

@@ -0,0 +1,3 @@
+# generated by datamodel-codegen:
+#   filename:  namespace_subns_ref.json
+#   timestamp: 1985-10-26T08:21:00+00:00

tests/data/expected/main/openapi/namespace_subns_ref/ns/__init__.py

@@ -0,0 +1,15 @@
+# generated by datamodel-codegen:
+#   filename:  namespace_subns_ref.json
+#   timestamp: 1985-10-26T08:21:00+00:00
+
+from __future__ import annotations
+
+from typing import Optional
+
+from pydantic import BaseModel
+
+from . import subns
+
+
+class Wrapper(BaseModel):
+    item: Optional[subns.Item] = None

tests/data/expected/main/openapi/namespace_subns_ref/ns/subns.py

@@ -0,0 +1,13 @@
+# generated by datamodel-codegen:
+#   filename:  namespace_subns_ref.json
+#   timestamp: 1985-10-26T08:21:00+00:00
+
+from __future__ import annotations
+
+from typing import Optional
+
+from pydantic import BaseModel
+
+
+class Item(BaseModel):
+    name: Optional[str] = None

tests/data/openapi/namespace_subns_ref.json

@@ -0,0 +1,28 @@
+{
+  "openapi": "3.0.1",
+  "info": {
+    "title": "Test API",
+    "version": "1.0.0"
+  },
+  "paths": {},
+  "components": {
+    "schemas": {
+      "ns.wrapper": {
+        "type": "object",
+        "properties": {
+          "item": {
+            "$ref": "#/components/schemas/ns.subns.item"
+          }
+        }
+      },
+      "ns.subns.item": {
+        "type": "object",
+        "properties": {
+          "name": {
+            "type": "string"
+          }
+        }
+      }
+    }
+  }
+}

tests/main/openapi/test_main_openapi.py

@@ -2401,3 +2401,19 @@ def test_main_openapi_external_ref_with_transitive_local_ref(output_file: Path)
         expected_file="external_ref_with_transitive_local_ref/output.py",
         extra_args=["--output-model-type", "pydantic_v2.BaseModel"],
     )
+
+
+def test_main_openapi_namespace_subns_ref(output_dir: Path) -> None:
+    """Test OpenAPI generation with namespaced schema referencing subnamespace.
+
+    Regression test for issue #2366: When a schema with a dot-delimited name
+    (e.g., ns.wrapper) references another schema in a subnamespace
+    (e.g., ns.subns.item), the generated import should be "from . import subns"
+    (same package) instead of "from .. import subns" (parent package).
+    """
+    with freeze_time(TIMESTAMP):
+        run_main_and_assert(
+            input_path=OPEN_API_DATA_PATH / "namespace_subns_ref.json",
+            output_path=output_dir,
+            expected_directory=EXPECTED_OPENAPI_PATH / "namespace_subns_ref",
+        )