Add omitempty to Node.ID (#89)

This commit adds omitempty to the field ID of type Node. This is needed
when constructing an entity client-side and sending it to the server for
creation. Without omitempty, the constructed entity will contain an
empty "id": "", which is not sensible to transmit.

This change addresses #83.

node.go

@@ -23,7 +23,7 @@ type ManyPayload struct {
 // Node is used to represent a generic JSON API Resource
 type Node struct {
 	Type          string                 `json:"type"`
-	ID            string                 `json:"id"`
+	ID            string                 `json:"id,omitempty"`
 	ClientID      string                 `json:"client-id,omitempty"`
 	Attributes    map[string]interface{} `json:"attributes,omitempty"`
 	Relationships map[string]interface{} `json:"relationships,omitempty"`

response_test.go

@@ -196,6 +196,32 @@ func TestMarshalIDPtr(t *testing.T) {
 	}
 }
 
+func TestMarshalOnePayload_omitIDString(t *testing.T) {
+	type Foo struct {
+		ID    string `jsonapi:"primary,foo"`
+		Title string `jsonapi:"attr,title"`
+	}
+
+	foo := &Foo{Title: "Foo"}
+	out := bytes.NewBuffer(nil)
+	if err := MarshalOnePayload(out, foo); 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{})
+
+	// Verify that empty ID of type string gets omitted. See:
+	// https://github.com/google/jsonapi/issues/83#issuecomment-285611425
+	_, ok := payload["id"]
+	if ok {
+		t.Fatal("Was expecting the data.id member to be omitted")
+	}
+}
+
 func TestMarshall_invalidIDType(t *testing.T) {
 	type badIDStruct struct {
 		ID *bool `jsonapi:"primary,cars"`