fix: handle deprecating relation for polyrelation

request.go

@@ -309,9 +309,33 @@ func unmarshalNode(data *Node, model reflect.Value, included *map[string]*Node)
 
 	modelValue := model.Elem()
 	modelType := modelValue.Type()
+	polyrelationFields := map[string]reflect.Type{}
 
 	var er error
 
+	// preprocess the model to find polyrelation fields
+	for i := 0; i < modelValue.NumField(); i++ {
+		fieldValue := modelValue.Field(i)
+		fieldType := modelType.Field(i)
+
+		args, err := getStructTags(fieldType)
+		if err != nil {
+			er = err
+			break
+		}
+
+		if len(args) < 2 {
+			continue
+		}
+
+		annotation := args[0]
+		name := args[1]
+
+		if annotation == annotationPolyRelation {
+			polyrelationFields[name] = fieldValue.Type()
+		}
+	}
+
 	for i := 0; i < modelValue.NumField(); i++ {
 		fieldValue := modelValue.Field(i)
 		fieldType := modelType.Field(i)
@@ -474,6 +498,13 @@ func unmarshalNode(data *Node, model reflect.Value, included *map[string]*Node)
 					continue
 				}
 
+				// If the field is also a polyrelation field, then prefer the polyrelation.
+				// Otherwise stop processing this node.
+				// This is to allow relation and polyrelation fields to coexist, supporting deprecation for consumers
+				if pFieldType, ok := polyrelationFields[args[1]]; ok && fieldValue.Type() != pFieldType {
+					continue
+				}
+
 				// This will hold either the value of the choice type model or the actual
 				// model, depending on annotation
 				m := reflect.New(fieldValue.Type().Elem())

request_test.go

@@ -847,6 +847,50 @@ func Test_UnmarshalPayload_polymorphicRelations_omitted(t *testing.T) {
 	}
 }
 
+func Test_UnmarshalPayload_polymorphicRelations_deprecatedRelation(t *testing.T) {
+	type withDeprecatedRelation struct {
+		ID    string      `jsonapi:"primary,blogs"`
+		Title string      `jsonapi:"attr,title"`
+		Media *OneOfMedia `jsonapi:"polyrelation,media"`
+		Image *Image      `jsonapi:"relation,media"` // Deprecated
+	}
+
+	in := bytes.NewReader([]byte(`{
+		"data": [{
+			"type": "blogs",
+			"id":   "3",
+			"attributes": {
+				"title": "Hello, World"
+			},
+			"relationships": {
+				"media": {
+					"data": {
+						"type": "videos",
+						"id":   "123"
+					}
+				}
+			}
+		}]
+	}`))
+
+	model := reflect.TypeOf(new(withDeprecatedRelation))
+
+	out, err := UnmarshalManyPayload(in, model)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	result := out[0].(*withDeprecatedRelation)
+
+	if result.Title != "Hello, World" {
+		t.Errorf("expected Title %q but got %q", "Hello, World", result.Title)
+	}
+
+	if result.Media.Video.ID != "123" {
+		t.Fatalf("expected Video to be \"123\", but got %+v", result.Media.Video)
+	}
+}
+
 func Test_choiceStructMapping(t *testing.T) {
 	cases := []struct {
 		val reflect.Type