tests: Add test case for nullable references

This is somewhat undefined behavior [1][2], however, it worked until
openapi-core 0.15.x. Demonstrate this, pending a fix.

[1] https://stackoverflow.com/a/48114924/613428
[2] OAI/OpenAPI-Specification#1368

Signed-off-by: Stephen Finucane <stephen@that.guru>

tests/integration/data/v3.0/nullable_ref.yaml

@@ -0,0 +1,58 @@
+openapi: "3.0.0"
+info:
+  version: "0.1"
+  title: OpenAPI specification with nullable refs
+paths:
+  /people/{personID}:
+    get:
+      summary: Show a person
+      parameters:
+        - name: personID
+          in: path
+          required: true
+          description: The ID of the person to retrieve
+          schema:
+            type: string
+            format: uuid
+      responses:
+        default:
+          description: Expected response to a valid request
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/Person'
+components:
+  schemas:
+    Person:
+      x-model: Person
+      type: object
+      required:
+        - id
+        - name
+      properties:
+        id:
+          description: The ID of the person.
+          type: string
+          format: uuid
+        name:
+          description: The full name of the person.
+          type: string
+        user:
+          description: The associated user account, if any.
+          nullable: true
+          allOf:
+            - $ref: '#/components/schemas/User'
+    User:
+      x-model: User
+      type: object
+      required:
+        - id
+        - username
+      properties:
+        id:
+          description: The ID of the user.
+          type: string
+          format: uuid
+        username:
+          description: The username of the user.
+          type: string

tests/integration/validation/test_nullable_ref.py

@@ -0,0 +1,62 @@
+from dataclasses import is_dataclass
+import json
+import uuid
+
+import pytest
+
+from openapi_core.testing import MockRequest
+from openapi_core.testing import MockResponse
+from openapi_core.validation.response import openapi_v30_response_validator
+
+
+@pytest.fixture(scope="class")
+def spec(factory):
+    return factory.spec_from_file("data/v3.0/nullable_ref.yaml")
+
+
+class TestNullableRefs:
+    @pytest.mark.xfail(message="The nullable attribute should be respected")
+    def test_with_null_value(self, spec):
+        person = {
+            "id": str(uuid.uuid4()),
+            "name": "Joe Bloggs",
+            "user": None,
+        }
+        request = MockRequest("", "get", f"/people/{person['id']}")
+        response = MockResponse(json.dumps(person))
+
+        result = openapi_v30_response_validator.validate(
+            spec, request, response
+        )
+
+        assert not result.errors
+        assert is_dataclass(result.data)
+        assert result.data.__class__.__name__ == "Person"
+        assert result.data.id == uuid.UUID(person["id"])
+        assert result.data.name == person["name"]
+        assert result.data.user is None
+
+    def test_with_non_null_value(self, spec):
+        person = {
+            "id": str(uuid.uuid4()),
+            "name": "Joe Bloggs",
+            "user": {
+                "id": str(uuid.uuid4()),
+                "username": "joebloggs",
+            },
+        }
+        request = MockRequest("", "get", f"/people/{person['id']}")
+        response = MockResponse(json.dumps(person))
+
+        result = openapi_v30_response_validator.validate(
+            spec, request, response
+        )
+
+        assert not result.errors
+        assert is_dataclass(result.data)
+        assert result.data.__class__.__name__ == "Person"
+        assert result.data.id == uuid.UUID(person["id"])
+        assert result.data.name == person["name"]
+        assert result.data.user is not None
+        assert result.data.user.id == uuid.UUID(person["user"]["id"])
+        assert result.data.user.username == person["user"]["username"]