// chats contain meta information about the chat like last message and count of unread messages

 let chatRef = ref.child("chats").child(receiverId).child(chatId)
 chatRef.observeSingleEventOfType(.Value, withBlock: { (snapshot) -> Void in

     if !snapshot.exists() {
         print("snapshot eists")
         // the other side has deleted the chat
         // now delete all messages and member objects

         ref.child("messages").child(chatId).setValue(nil)
         ref.child("members").child(chatId).setValue(nil)
     } else {
         print("snapshot does not exist")
     }
 })