自动推送新的聊天消息并更新Firebase实时数据库上的lastMessage值

Question

我将聊天消息和聊天元数据存储在此结构中：

project
|
-- chats
   |
   -- 1
      |
      -- -MFBvnTIQgVKdyFzDMpy
         |
         -- message: "hello"
      |
      -- -MEZutiaxOthE-7nDOkA
         |
         -- message: "how are you?"
   |
   -- 2
      |
      -- -MENuu8TjwWrBTkIzue_
         |
         -- message: "hi"
      |
      -- -MFBTqEqhR9Dtv3MlMd6
         |
         -- message: "you good?"
|
-- chatMetadata
   |
   -- 1
      |
      -- lastMessage: "how are you?"
   |
   -- 2
      |
      -- lastMessage: "you good?"

每当用户向特定聊天提交新的聊天消息时，该聊天的lastMessage属性都应该在chatMetadata中更新。流程包含两个独立的操作:推送新聊天消息的.push()操作和更新lastMessage属性的.update()操作。如果多个用户可以同时向某个聊天提交消息，如何保证lastMessage总是等于上次推送的聊天消息？

Answer 1

为此，您必须使用云函数的实时数据库触发器。

// Listens for new messages added to /chats/:chatId/ and creates an
// uppercase version of the message to /chatMetadata/:chatId/

exports.updateLastMessage = functions.database.ref('/chats/{chatId}/')
    .onCreate((snapshot, context) => {
      // Grab the current value of what was written to the Realtime Database.
      const original = snapshot.val();
      
      // You must return a Promise when performing asynchronous tasks inside a Functions 
      // Writing the lastMessage node 
      return snapshot.ref.parent.parent.child('chatMetadata').child(context.params.chatId).set({lastMessage: original.message});
});