alert mute cannot refresh the bug (#1242)

bugfix:mute cannot be refreshed after being modified

src/models/alert_mute.go

@@ -174,7 +174,7 @@ func AlertMuteStatistics(cluster string) (*Statistics, error) {
 		return nil, err
 	}
 
-	session := DB().Model(&AlertMute{}).Select("count(*) as total", "max(create_at) as last_updated")
+	session := DB().Model(&AlertMute{}).Select("count(*) as total", "max(update_at) as last_updated")
 	if cluster != "" {
 		session = session.Where("(cluster like ? or cluster = ?)", "%"+cluster+"%", ClusterAll)
 	}