Fix custom Panel JSON marshalling (#167)

Author: krya-kryak

panel.go

@@ -20,6 +20,7 @@ package sdk
 */
 
 import (
+	"bytes"
 	"encoding/json"
 	"errors"
 )
@@ -1053,15 +1054,42 @@ func (p *Panel) MarshalJSON() ([]byte, error) {
 		}{p.CommonPanel, *p.HeatmapPanel}
 		return json.Marshal(outHeatmap)
 	case CustomType:
-		var outCustom = struct {
-			CommonPanel
-			CustomPanel
-		}{p.CommonPanel, *p.CustomPanel}
+		var outCustom = customPanelOutput{
+			p.CommonPanel,
+			*p.CustomPanel,
+		}
 		return json.Marshal(outCustom)
 	}
 	return nil, errors.New("can't marshal unknown panel type")
 }
 
+type customPanelOutput struct {
+	CommonPanel
+	CustomPanel
+}
+
+func (c customPanelOutput) MarshalJSON() ([]byte, error) {
+	b, err := json.Marshal(c.CommonPanel)
+	if err != nil {
+		return b, err
+	}
+	// Append custom keys to marshalled CommonPanel.
+	buf := bytes.NewBuffer(b[:len(b)-1])
+
+	for k, v := range c.CustomPanel {
+		buf.WriteString(`,"`)
+		buf.WriteString(k)
+		buf.WriteString(`":`)
+		b, err := json.Marshal(v)
+		if err != nil {
+			return b, err
+		}
+		buf.Write(b)
+	}
+	buf.WriteString("}")
+	return buf.Bytes(), nil
+}
+
 func incRefID(refID string) string {
 	firstLetter := refID[0]
 	ordinal := int(firstLetter)

panel_test.go

@@ -695,3 +695,42 @@ func TestPanel_Stackdriver_ParsedTargets(t *testing.T) {
 		t.Fatalf("should be \"pubsub.googleapis.com/subscription/ack_message_count\" but is not")
 	}
 }
+
+// TestCustomPanelOutput_MarshalJSON marshals new custom panel to JSON,
+// then marshals that json to map[string]interface{},\
+// and then checks both custom and non-custom keys are present and correct.
+func TestCustomPanelOutput_MarshalJSON(t *testing.T) {
+	var (
+		titleKey    = "title"
+		titleValue  = "test title"
+		customKey   = "test_key"
+		customValue = "bar_value"
+	)
+	p := sdk.NewCustom(titleValue)
+	custom := map[string]interface{}(*p.CustomPanel)
+	custom[customKey] = customValue
+	b, err := json.Marshal(p)
+	if err != nil {
+		t.Fatalf("failed to marshal custom panel: %v", err)
+	}
+	var j = make(map[string]interface{})
+	err = json.Unmarshal(b, &j)
+	if err != nil {
+		t.Fatalf("failed to unmarshal: %v", err)
+	}
+	val, ok := j[customKey]
+	if !ok {
+		t.Fatalf("failed to find key %s in map %v", customKey, j)
+	}
+	if val != customValue {
+		t.Fatalf("wrong value of %s: got %s, expected %s", customKey, val, customValue)
+	}
+	val, ok = j[titleKey]
+	if !ok {
+		t.Fatalf("failed to find key %s in map %v", titleKey, j)
+	}
+	if val != titleValue {
+		t.Fatalf("wrong value of %s: got %s, expected %s", titleKey, val, titleValue)
+	}
+
+}