Fix node id marshalling for custom string types

request.go

@@ -542,9 +542,29 @@ func fullNode(n *Node, included *map[string]*Node) *Node {
 // assign will take the value specified and assign it to the field; if
 // field is expecting a ptr assign will assign a ptr.
 func assign(field, value reflect.Value) {
-	if field.Kind() == reflect.Ptr {
+	if value.Type().AssignableTo(field.Type()) {
 		field.Set(value)
+		return
+	}
+
+	indirectValue := reflect.Indirect(value)
+	if indirectValue.Type().AssignableTo(field.Type()) {
+		field.Set(indirectValue)
+		return
+	}
+
+	// Conversion required.
+
+	if field.Kind() == reflect.Ptr {
+		if value.Kind() == reflect.Ptr {
+			field.Set(value.Convert(field.Type()))
+		} else {
+			// Because field is zero value, we cannot simply field.Elem().Set().
+			v := reflect.New(field.Type().Elem())
+			v.Elem().Set(value.Convert(field.Type().Elem()))
+			field.Set(v)
+		}
 	} else {
-		field.Set(reflect.Indirect(value))
+		field.Set(reflect.Indirect(value).Convert(field.Type()))
 	}
 }

request_test.go

@@ -703,6 +703,112 @@ func TestManyPayload_withLinks(t *testing.T) {
 	}
 }
 
+func TestUnmarshalPayloadIDTypeOfString(t *testing.T) {
+	t.Run("Unmarshal string to value type", func(t *testing.T) {
+		data := map[string]interface{}{
+			"data": map[string]interface{}{
+				"type": "books",
+				"id":   "978-3-16-148410-0",
+				"attributes": map[string]interface{}{
+					"title": "Gesammelte Werke in deutscher Sprache",
+				},
+			},
+		}
+		b, err := json.Marshal(data)
+		if err != nil {
+			t.Fatalf("Unexpected error: %v", err)
+		}
+
+		type (
+			IBSN string
+
+			Book struct {
+				ID    IBSN   `jsonapi:"primary,books"`
+				Title string `jsonapi:"attr,title"`
+			}
+		)
+
+		book := &Book{}
+		if err := UnmarshalPayload(bytes.NewReader(b), book); err != nil {
+			t.Fatalf("Unexpected error: %v", err)
+		}
+
+		expected := IBSN("978-3-16-148410-0")
+		if book.ID != expected {
+			t.Fatalf("Expected book id to be %v but got %v", expected, book.ID)
+		}
+	})
+
+	t.Run("Unmarshal string to ptr type", func(t *testing.T) {
+		data := map[string]interface{}{
+			"data": map[string]interface{}{
+				"type": "books",
+				"id":   "978-3-16-148410-0",
+				"attributes": map[string]interface{}{
+					"title": "Gesammelte Werke in deutscher Sprache",
+				},
+			},
+		}
+		b, err := json.Marshal(data)
+		if err != nil {
+			t.Fatalf("Unexpected error: %v", err)
+		}
+
+		type (
+			IBSN string
+
+			Book struct {
+				ID    *IBSN  `jsonapi:"primary,books"`
+				Title string `jsonapi:"attr,title"`
+			}
+		)
+
+		book := &Book{}
+		if err := UnmarshalPayload(bytes.NewReader(b), book); err != nil {
+			t.Fatalf("Unexpected error: %v", err)
+		}
+
+		expected := IBSN("978-3-16-148410-0")
+		if !reflect.DeepEqual(book.ID, &expected) {
+			t.Fatalf("Expected book id to be %v but got %v", &expected, book.ID)
+		}
+	})
+
+	t.Run("Unmarshal nil to ptr type", func(t *testing.T) {
+		data := map[string]interface{}{
+			"data": map[string]interface{}{
+				"type": "books",
+				"id":   nil,
+				"attributes": map[string]interface{}{
+					"title": "Gesammelte Werke in deutscher Sprache",
+				},
+			},
+		}
+		b, err := json.Marshal(data)
+		if err != nil {
+			t.Fatalf("Unexpected error: %v", err)
+		}
+
+		type (
+			IBSN string
+
+			Book struct {
+				ID    *IBSN  `jsonapi:"primary,books"`
+				Title string `jsonapi:"attr,title"`
+			}
+		)
+
+		book := &Book{}
+		if err := UnmarshalPayload(bytes.NewReader(b), book); err != nil {
+			t.Fatalf("Unexpected error: %v", err)
+		}
+
+		if book.ID != nil {
+			t.Fatalf("Expected book id to be %v but got %v", nil, book.ID)
+		}
+	})
+}
+
 func samplePayloadWithoutIncluded() map[string]interface{} {
 	return map[string]interface{}{
 		"data": map[string]interface{}{

response.go

@@ -251,7 +251,7 @@ func visitModelNode(model interface{}, included *map[string]*Node,
 			// Handle allowed types
 			switch kind {
 			case reflect.String:
-				node.ID = v.Interface().(string)
+				node.ID = v.String()
 			case reflect.Int:
 				node.ID = strconv.FormatInt(int64(v.Interface().(int)), 10)
 			case reflect.Int8:

response_test.go

@@ -279,6 +279,38 @@ func TestMarshalIDPtr(t *testing.T) {
 	}
 }
 
+func TestMarshalIDTypeOfString(t *testing.T) {
+	type (
+		IBSN string
+
+		Book struct {
+			ID IBSN `jsonapi:"primary,books"`
+		}
+	)
+
+	book := &Book{ID: IBSN("978-3-16-148410-0")}
+
+	out := &bytes.Buffer{}
+	if err := MarshalPayload(out, book); err != nil {
+		t.Fatal(err)
+	}
+
+	var payload map[string]interface{}
+	if err := json.Unmarshal(out.Bytes(), &payload); err != nil {
+		t.Fatal(err)
+	}
+
+	data := payload["data"].(map[string]interface{})
+	id, ok := data["id"]
+	if !ok {
+		t.Fatal("Was expecting the data.id value to exist")
+	}
+
+	if id != "978-3-16-148410-0" {
+		t.Fatalf("Was expecting the data.id value to be %s but got %s", "978-3-16-148410-0", id)
+	}
+}
+
 func TestMarshalOnePayload_omitIDString(t *testing.T) {
 	type Foo struct {
 		ID    string `jsonapi:"primary,foo"`