Mongoose returned array after saving

I am trying to return an updated object as JSON where the update was supposed to set an array of object ids. I want the returned object to object to this array being full. For example, I have the following (simplified) model:

var UserSchema = new mongoose.Schema({
  username:     {type: String, unique: true, required: true},
  friends:      [{type: mongoose.Schema.Types.ObjectId, ref: 'User'}]
});

      

        
        
        
      

    

In my controller, I have:

exports.saveFriends = function(req, res) {
  User.findById(req.params.user_id, function(err, user) {

    // req.body.friends is JSON list of objectIDs for other users
    user.friends = req.body.friends

    user.save(function(err) {
      user.populate({path: 'friends'}, function(err, ticket) {
        if (err) {
          res.send(err);
        } else {
          res.json(user);
        }
      });
    });
  });
}

      

        
        
        
      

    

This actually stores the array properly as the ObjectID, but the user of the response always shows "[]" as an array of friends.

Does anyone see my problem?