Fix nullable nested schemas with metadata..

#955

Applied the same `allOf` logic seen used in 3.1 (See
#904)

src/apispec/ext/marshmallow/field_converter.py

@@ -306,6 +306,11 @@ def field2nullable(self, field: marshmallow.fields.Field, ret) -> dict:
                         {"type": "object", "nullable": True},
                         {"$ref": ret.pop("$ref")},
                     ]
+                elif "allOf" in ret:
+                    attributes["anyOf"] = [
+                        *ret.pop("allOf"),
+                        {"type": "object", "nullable": True},
+                    ]
                 else:
                     attributes["nullable"] = True
             else:

tests/test_ext_marshmallow_field.py

@@ -222,6 +222,42 @@ class Child(Schema):
         }
 
 
+@pytest.mark.parametrize("spec_fixture", ("2.0", "3.0.0", "3.1.0"), indirect=True)
+def test_nested_nullable_with_metadata(spec_fixture):
+    # Regression test for https://github.com/marshmallow-code/apispec/issues/955
+    class Child(Schema):
+        name = fields.Str()
+
+    field = fields.Nested(
+        Child,
+        allow_none=True,
+        metadata={"description": "foo"},
+    )
+    res = spec_fixture.openapi.field2property(field)
+    version = spec_fixture.openapi.openapi_version
+    if version.major < 3:
+        assert res == {
+            "allOf": [
+                {"$ref": "#/definitions/Child"},
+            ],
+            "x-nullable": True,
+            "description": "foo",
+        }
+    elif version.major == 3 and version.minor < 1:
+        assert res == {
+            "anyOf": [
+                {"$ref": "#/components/schemas/Child"},
+                {"type": "object", "nullable": True},
+            ],
+            "description": "foo",
+        }
+    else:
+        assert res == {
+            "anyOf": [{"$ref": "#/components/schemas/Child"}, {"type": "null"}],
+            "description": "foo",
+        }
+
+
 @pytest.mark.parametrize("spec_fixture", ("2.0", "3.0.0", "3.1.0"), indirect=True)
 def test_nullable_pluck(spec_fixture):
     class Example(Schema):