您如何使用node.js在猫鼬（mongodb）中的数组上进行“联接”？

Question

How do you do a "join" (i know it is not the correct term) with an array of messages in mongoose?

I tried looping over all the messages and querying to get the user info but it is not working:

messages.forEach(function (message, index) {
  User.findById(message.userId, function (err, user) {
    messages[index].user = user
  })
})

console.log(messages) // the user info is not attatched

So how is this accomplished with mongoose and node.js?

Answer 1

the biggest problem with your code is, that you assume the code to run synchronously - but it doesn't. it runs asynchronously. so messages is not yet set when you execute

 console.log(messages);

do something like this instead:

var userIds = [id1, id2, id3];
User.find({"_id": {$in: userIds}}, function (err, users) {
  console.log(users);
});

edit ok, i see. you want to add the userInfo to the different messages. easiest way to acieve this, is to use the async module: https://github.com/caolan/async

async.map(messages, getUserInfo, function (err, result) {
  if (err) {
    console.log(err);
    return;
  }
  // log all msg with userinfo
  console.log(result);
});

function getUserInfo (msg, callback) {
  User.findById(msg.userId, function (err, user) {
    if (err) {
       callback(err);
       return;
    }
    msg.user = user;
    callback(null, msg);
  });
}