Fix nullable propagation from $ref schema targets (#2649)

* Fix nullable reference propagation in JSON Schema and OpenAPI handling

* [pre-commit.ci] auto fixes from pre-commit.com hooks

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* Fix formatting inconsistencies in test cases for JSON Schema handling

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

* Refactor get_mock_response function to use underscore for unused kwargs

* Refactor get_mock_response function to use underscore for unused kwargs

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

Author: koxudaxi

src/datamodel_code_generator/parser/jsonschema.py

@@ -1038,7 +1038,11 @@ def _get_data_type(type_: str, format__: str) -> DataType:
     def get_ref_data_type(self, ref: str) -> DataType:
         """Get a data type from a reference string."""
         reference = self.model_resolver.add_ref(ref)
-        return self.data_type(reference=reference)
+        ref_schema = self._load_ref_schema_object(ref)
+        is_optional = (
+            ref_schema.type_has_null or ref_schema.type == "null" or (self.strict_nullable and ref_schema.nullable)
+        )
+        return self.data_type(reference=reference, is_optional=is_optional)
 
     def set_additional_properties(self, path: str, obj: JsonSchemaObject) -> None:
         """Set additional properties flag in extra template data."""

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

@@ -0,0 +1,25 @@
+# generated by datamodel-codegen:
+#   filename:  ref_type_has_null.json
+#   timestamp: 2019-07-26T00:00:00+00:00
+
+from __future__ import annotations
+
+from pydantic import BaseModel
+
+
+class NullableString(BaseModel):
+    __root__: str | None
+
+
+class NonNullableString(BaseModel):
+    __root__: str
+
+
+class NullOnly(BaseModel):
+    __root__: None
+
+
+class Model(BaseModel):
+    nullableRef: NullableString | None
+    nonNullableRef: NonNullableString
+    nullOnlyRef: NullOnly | None

tests/data/expected/main/openapi/ref_nullable_strict_nullable.py

@@ -0,0 +1,20 @@
+# generated by datamodel-codegen:
+#   filename:  ref_nullable.yaml
+#   timestamp: 2019-07-26T00:00:00+00:00
+
+from __future__ import annotations
+
+from pydantic import BaseModel
+
+
+class NullableChild(BaseModel):
+    name: str | None = None
+
+
+class NonNullableChild(BaseModel):
+    name: str | None = None
+
+
+class Parent(BaseModel):
+    nullableChild: NullableChild | None
+    nonNullableChild: NonNullableChild

tests/data/jsonschema/ref_type_has_null.json

@@ -0,0 +1,27 @@
+{
+  "$schema": "http://json-schema.org/draft-07/schema#",
+  "definitions": {
+    "NullableString": {
+      "type": ["string", "null"]
+    },
+    "NonNullableString": {
+      "type": "string"
+    },
+    "NullOnly": {
+      "type": "null"
+    }
+  },
+  "type": "object",
+  "required": ["nullableRef", "nonNullableRef", "nullOnlyRef"],
+  "properties": {
+    "nullableRef": {
+      "$ref": "#/definitions/NullableString"
+    },
+    "nonNullableRef": {
+      "$ref": "#/definitions/NonNullableString"
+    },
+    "nullOnlyRef": {
+      "$ref": "#/definitions/NullOnly"
+    }
+  }
+}

tests/data/openapi/ref_nullable.yaml

@@ -0,0 +1,28 @@
+openapi: 3.0.0
+info:
+  title: Test API
+  version: 1.0.0
+paths: {}
+components:
+  schemas:
+    NullableChild:
+      nullable: true
+      type: object
+      properties:
+        name:
+          type: string
+    NonNullableChild:
+      type: object
+      properties:
+        name:
+          type: string
+    Parent:
+      type: object
+      required:
+        - nullableChild
+        - nonNullableChild
+      properties:
+        nullableChild:
+          $ref: "#/components/schemas/NullableChild"
+        nonNullableChild:
+          $ref: "#/components/schemas/NonNullableChild"