Question

I’m serving a JSON API for posts, where I want to deliver the post object after linking it to the appropriate user model.

I’m having a MongoDB schema like this:

User = {
    email: String,
    password: String
};

Post = {
    user: ObjectId,
    text: String
};

I don’t find a problem linking a single post, which I’ve done using the following code (I’m using Mongoose and Express):

app.get('/api/posts/:id', function(req, res){
  return Post.findById(req.params.id, function(err, post) {
    if (!err) {
        User.findById(post.user, function(err, user){

            var joinedpost = {
                text : post.text,
                user : user
            };
          return res.send(joinedpost);      
        });
     }
  });
});

However, when I want to server multiple posts I face the following problem: Since I’m working in an asynchronous environment I can’t simply fetch all posts and iterate over them to link each post to its user.

So I think I should write two queries, one that fetches the posts, and the other links them all at once, so that I can serve them immediately. Am I correct? And if so, how can I achieve that?

Answer 1

The $in operator will let you do this in one query. Here’s how I’d do it (assuming you use underscore):

app.get('/api/posts', function(req, res){
  return Post.find(function(err, posts) {
    if (!err) {
      var userIds = _.pluck(posts, user);
      User.find({"id": {"$in": userIds}}, function(err, users){
        var map = {};
        _.each(users, function(u){
          map[u.id] = u;
        });
        _.each(posts, function(p){
          p.user = map[p.user];
        });
        return res.send(posts);
      });
    });
   }
});

This isn’t well tested, and you’d need to handle the error condition on the User.find call, but it should be a good starting point.

Also, the async library helps a lot with stuff like this. You might look into that and something like the waterfall method to get around the crazy nested callback problem.