fix: missing receivers in json payload for legacy postableAlert (#7603)

pkg/alertmanager/legacyalertmanager/provider.go

@@ -25,6 +25,25 @@ type postableAlert struct {
 	Receivers []string `json:"receivers"`
 }
 
+func (pa *postableAlert) MarshalJSON() ([]byte, error) {
+	// Marshal the embedded PostableAlert to get its JSON representation.
+	alertJSON, err := json.Marshal(pa.PostableAlert)
+	if err != nil {
+		return nil, err
+	}
+
+	// Unmarshal that JSON into a map so we can add extra fields.
+	var m map[string]interface{}
+	if err := json.Unmarshal(alertJSON, &m); err != nil {
+		return nil, err
+	}
+
+	// Add the Receivers field.
+	m["receivers"] = pa.Receivers
+
+	return json.Marshal(m)
+}
+
 const (
 	alertsPath       string = "/v1/alerts"
 	routesPath       string = "/v1/routes"

pkg/alertmanager/legacyalertmanager/provider_test.go

@@ -0,0 +1,35 @@
+package legacyalertmanager
+
+import (
+	"encoding/json"
+	"testing"
+
+	"github.com/SigNoz/signoz/pkg/types/alertmanagertypes"
+	"github.com/prometheus/alertmanager/api/v2/models"
+	"github.com/stretchr/testify/assert"
+)
+
+func TestProvider_TestAlert(t *testing.T) {
+	pa := &postableAlert{
+		PostableAlert: &alertmanagertypes.PostableAlert{
+			Alert: models.Alert{
+				Labels: models.LabelSet{
+					"alertname": "test",
+				},
+				GeneratorURL: "http://localhost:9090/graph?g0.expr=up&g0.tab=1",
+			},
+			Annotations: models.LabelSet{
+				"summary": "test",
+			},
+		},
+		Receivers: []string{"receiver1", "receiver2"},
+	}
+
+	body, err := json.Marshal(pa)
+	if err != nil {
+		t.Fatalf("failed to marshal postable alert: %v", err)
+	}
+
+	assert.Contains(t, string(body), "receiver1")
+	assert.Contains(t, string(body), "receiver2")
+}