add custom handling of NullFields to marshaling

response.go

@@ -233,6 +233,12 @@ func visitModelNode(model interface{}, included *map[string]*Node,
 
 	modelValue := value.Elem()
 	modelType := value.Type().Elem()
+	nullFields := []string{}
+	if _, ok := modelType.FieldByName("NullFields"); ok {
+		if modelValue.FieldByName("NullFields").Kind() == reflect.Slice {
+			nullFields = modelValue.FieldByName("NullFields").Interface().([]string)
+		}
+	}
 
 	for i := 0; i < modelValue.NumField(); i++ {
 		fieldValue := modelValue.Field(i)
@@ -348,7 +354,7 @@ func visitModelNode(model interface{}, included *map[string]*Node,
 			} else if fieldValue.Type() == reflect.TypeOf(new(time.Time)) {
 				// A time pointer may be nil
 				if fieldValue.IsNil() {
-					if omitEmpty {
+					if omitEmpty && !stringInSlice(nullFields, structField.Name) {
 						continue
 					}
 
@@ -373,7 +379,7 @@ func visitModelNode(model interface{}, included *map[string]*Node,
 				emptyValue := reflect.Zero(fieldValue.Type())
 
 				// See if we need to omit this field
-				if omitEmpty && reflect.DeepEqual(fieldValue.Interface(), emptyValue.Interface()) {
+				if omitEmpty && reflect.DeepEqual(fieldValue.Interface(), emptyValue.Interface()) && !stringInSlice(nullFields, structField.Name) {
 					continue
 				}
 
@@ -648,3 +654,12 @@ func convertToSliceInterface(i *interface{}) ([]interface{}, error) {
 	}
 	return response, nil
 }
+
+func stringInSlice(s []string, v string) bool {
+	for _, field := range s {
+		if field == v {
+			return true
+		}
+	}
+	return false
+}

response_test.go

@@ -518,6 +518,47 @@ func TestWithOmitsEmptyAnnotationOnAttribute(t *testing.T) {
 	}
 }
 
+func TestOmitsEmptyAnnotationAndNullFieldOnAttribute(t *testing.T) {
+	type Author struct{}
+
+	type Book struct {
+		ID          int        `jsonapi:"primary,books"`
+		Name        string     `jsonapi:"attr,name"`
+		Author      *Author    `jsonapi:"attr,author,omitempty"`
+		PublishedAt *time.Time `jsonapi:"attr,published_at,omitempty"`
+
+		NullFields []string
+	}
+
+	book := &Book{
+		ID:          123,
+		Name:        "The Confidence Man",
+		PublishedAt: nil,
+
+		NullFields: []string{"PublishedAt"},
+	}
+
+	out := bytes.NewBuffer(nil)
+	if err := MarshalPayload(out, book); err != nil {
+		t.Fatal(err)
+	}
+
+	var jsonData map[string]interface{}
+	if err := json.Unmarshal(out.Bytes(), &jsonData); err != nil {
+		t.Fatal(err)
+	}
+
+	payload := jsonData["data"].(map[string]interface{})
+	attributes := payload["attributes"].(map[string]interface{})
+	if _, ok := attributes["published_at"]; !ok {
+		t.Fatal("Was expecting the data.attributes.published_at to have NOT been omitted")
+	}
+
+	if _, ok := attributes["author"]; ok {
+		t.Fatal("Was expecting the data.attributes.author to have been omitted")
+	}
+}
+
 func TestMarshalIDPtr(t *testing.T) {
 	id, make, model := "123e4567-e89b-12d3-a456-426655440000", "Ford", "Mustang"
 	car := &Car{