Fix omitted model value for polyrelation fields

Author: brandonc

request_test.go

@@ -734,6 +734,37 @@ func Test_UnmarshalPayload_polymorphicRelations_no_choice(t *testing.T) {
 	}
 }
 
+func Test_UnmarshalPayload_polymorphicRelations_omitted(t *testing.T) {
+	type pointerToOne struct {
+		ID    string      `jsonapi:"primary,blogs"`
+		Title string      `jsonapi:"attr,title"`
+		Hero  *OneOfMedia `jsonapi:"polyrelation,hero-media"`
+	}
+
+	in := bytes.NewReader([]byte(`{
+		"data": {
+			"type": "blogs",
+			"id":   "3",
+			"attributes": {
+				"title": "Hello, World"
+			}
+		}
+	}`))
+	out := new(pointerToOne)
+
+	if err := UnmarshalPayload(in, out); err != nil {
+		t.Fatal(err)
+	}
+
+	if out.Title != "Hello, World" {
+		t.Errorf("expected Title %q but got %q", "Hello, World", out.Title)
+	}
+
+	if out.Hero != nil {
+		t.Fatalf("expected Hero to be nil, but got %+v", out.Hero)
+	}
+}
+
 func Test_choiceStructMapping(t *testing.T) {
 	cases := []struct {
 		val reflect.Type

response.go

@@ -417,8 +417,13 @@ func visitModelNode(model interface{}, included *map[string]*Node,
 					if choiceValue.IsNil() {
 						fieldValue = reflect.ValueOf(nil)
 					}
-
 					structValue := choiceValue.Elem()
+
+					// Short circuit if field is omitted from model
+					if !structValue.IsValid() {
+						break
+					}
+
 					if found, err := selectChoiceTypeStructField(structValue); err == nil {
 						fieldValue = found
 					}

response_test.go

@@ -167,6 +167,18 @@ func TestMarshalPayloadWithHasOneNilPolyrelation(t *testing.T) {
 	}
 }
 
+func TestMarshalPayloadWithHasOneOmittedPolyrelation(t *testing.T) {
+	blog := &BlogPostWithPoly{
+		ID:    "1",
+		Title: "Hello, World",
+	}
+
+	out := bytes.NewBuffer(nil)
+	if err := MarshalPayload(out, blog); err != nil {
+		t.Fatalf("expected no error but got %s", err)
+	}
+}
+
 func TestMarshalPayloadWithHasOneNilRelation(t *testing.T) {
 	blog := &Blog{
 		ID:    1,