Correct import generation for dot notation schema names with inheritance (#2607)

* Fix: Enhance ancestor package reference resolution for imports in dot notation schemas

* Fix: Simplify ancestor package reference handling in import resolution

Author: koxudaxi

src/datamodel_code_generator/parser/base.py

@@ -293,6 +293,45 @@ def relative(current_module: str, reference: str) -> tuple[str, str]:
     return left, right
 
 
+def is_ancestor_package_reference(current_module: str, reference: str) -> bool:
+    """Check if reference is in an ancestor package (__init__.py).
+
+    When the reference's module path is an ancestor (prefix) of the current module,
+    the reference is in an ancestor package's __init__.py file.
+
+    Args:
+        current_module: The current module path (e.g., "v0.mammal.canine")
+        reference: The full reference path (e.g., "v0.Animal")
+
+    Returns:
+        True if the reference is in an ancestor package, False otherwise.
+
+    Examples:
+        - current="v0.animal", ref="v0.Animal" -> True (immediate parent)
+        - current="v0.mammal.canine", ref="v0.Animal" -> True (grandparent)
+        - current="v0.animal", ref="v0.animal.Dog" -> False (same or child)
+        - current="pets", ref="Animal" -> True (root package is immediate parent)
+    """
+    current_path = current_module.split(".") if current_module else []
+    *reference_path, _ = reference.split(".")
+
+    if not current_path:
+        return False
+
+    # Case 1: Direct parent package (includes root package when reference_path is empty)
+    # e.g., current="pets", ref="Animal" -> current_path[:-1]=[] == reference_path=[]
+    if current_path[:-1] == reference_path:
+        return True
+
+    # Case 2: Deeper ancestor package (reference_path must be non-empty proper prefix)
+    # e.g., current="v0.mammal.canine", ref="v0.Animal" -> ["v0"] is prefix of ["v0","mammal","canine"]
+    return (
+        len(reference_path) > 0
+        and len(reference_path) < len(current_path)
+        and current_path[: len(reference_path)] == reference_path
+    )
+
+
 def exact_import(from_: str, import_: str, short_name: str) -> tuple[str, str]:
     """Create exact import path to avoid relative import issues."""
     if from_ == len(from_) * ".":
@@ -813,7 +852,8 @@ def __change_from_import(
 
                 if isinstance(data_type, BaseClassDataType):
                     left, right = relative(model.module_name, data_type.full_name)
-                    from_ = f"{left}{right}" if left.endswith(".") else f"{left}.{right}"
+                    is_ancestor = is_ancestor_package_reference(model.module_name, data_type.full_name)
+                    from_ = left if is_ancestor else (f"{left}{right}" if left.endswith(".") else f"{left}.{right}")
                     import_ = data_type.reference.short_name
                     full_path = from_, import_
                 else:

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

@@ -0,0 +1,3 @@
+# generated by datamodel-codegen:
+#   filename:  dot_notation_deep_inheritance.yaml
+#   timestamp: 2019-07-26T00:00:00+00:00

tests/data/expected/main/openapi/dot_notation_deep_inheritance/v0/__init__.py

@@ -0,0 +1,13 @@
+# generated by datamodel-codegen:
+#   filename:  dot_notation_deep_inheritance.yaml
+#   timestamp: 2019-07-26T00:00:00+00:00
+
+from __future__ import annotations
+
+from typing import Optional
+
+from pydantic import BaseModel
+
+
+class Animal(BaseModel):
+    species: Optional[str] = None

tests/data/expected/main/openapi/dot_notation_deep_inheritance/v0/mammal/__init__.py

@@ -0,0 +1,13 @@
+# generated by datamodel-codegen:
+#   filename:  dot_notation_deep_inheritance.yaml
+#   timestamp: 2019-07-26T00:00:00+00:00
+
+from __future__ import annotations
+
+from typing import Optional
+
+from .. import Animal
+
+
+class Dog(Animal):
+    breed: Optional[str] = None

tests/data/expected/main/openapi/dot_notation_deep_inheritance/v0/mammal/canine.py

@@ -0,0 +1,13 @@
+# generated by datamodel-codegen:
+#   filename:  dot_notation_deep_inheritance.yaml
+#   timestamp: 2019-07-26T00:00:00+00:00
+
+from __future__ import annotations
+
+from typing import Optional
+
+from .. import Animal
+
+
+class Puppy(Animal):
+    age_weeks: Optional[int] = None

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

@@ -0,0 +1,3 @@
+# generated by datamodel-codegen:
+#   filename:  dot_notation_inheritance.yaml
+#   timestamp: 2019-07-26T00:00:00+00:00

tests/data/expected/main/openapi/dot_notation_inheritance/v0/__init__.py

@@ -0,0 +1,18 @@
+# generated by datamodel-codegen:
+#   filename:  dot_notation_inheritance.yaml
+#   timestamp: 2019-07-26T00:00:00+00:00
+
+from __future__ import annotations
+
+from typing import Optional
+
+from pydantic import BaseModel
+
+
+class Properties(BaseModel):
+    name: Optional[str] = None
+
+
+class Animal(BaseModel):
+    species: Optional[str] = None
+    props: Optional[Properties] = None

tests/data/expected/main/openapi/dot_notation_inheritance/v0/animal.py

@@ -0,0 +1,13 @@
+# generated by datamodel-codegen:
+#   filename:  dot_notation_inheritance.yaml
+#   timestamp: 2019-07-26T00:00:00+00:00
+
+from __future__ import annotations
+
+from typing import Optional
+
+from . import Animal
+
+
+class Dog(Animal):
+    breed: Optional[str] = None

tests/data/openapi/dot_notation_deep_inheritance.yaml

@@ -0,0 +1,29 @@
+openapi: "3.0.0"
+info:
+  title: Test API with deep inheritance from ancestor packages
+  version: "1.0.0"
+paths: {}
+components:
+  schemas:
+    v0.Animal:
+      type: object
+      description: Base animal in root package
+      properties:
+        species:
+          type: string
+    v0.mammal.Dog:
+      description: Dog inheriting from Animal (2 levels up)
+      allOf:
+        - $ref: '#/components/schemas/v0.Animal'
+        - type: object
+          properties:
+            breed:
+              type: string
+    v0.mammal.canine.Puppy:
+      description: Puppy inheriting from Animal (3 levels up)
+      allOf:
+        - $ref: '#/components/schemas/v0.Animal'
+        - type: object
+          properties:
+            age_weeks:
+              type: integer

tests/data/openapi/dot_notation_inheritance.yaml

@@ -0,0 +1,29 @@
+openapi: "3.0.0"
+info:
+  title: Test API with dot notation schema names and inheritance
+  version: "1.0.0"
+paths: {}
+components:
+  schemas:
+    v0.properties:
+      type: object
+      description: Base properties schema
+      properties:
+        name:
+          type: string
+    v0.animal:
+      type: object
+      description: Animal schema that references v0.properties
+      properties:
+        species:
+          type: string
+        props:
+          $ref: '#/components/schemas/v0.properties'
+    v0.animal.dog:
+      description: Dog schema that extends v0.animal
+      allOf:
+        - $ref: '#/components/schemas/v0.animal'
+        - type: object
+          properties:
+            breed:
+              type: string