es6 mongoose nested findById promise - arrays

Question

I'm building a restful api using node express mongoose/mongo etc.. I'm trying to output an array of users that are being followed by a particular user. Here is the schema.
var UserSchema = new mongoose.Schema({
username: {type: String, lowercase: true, unique: true, required: [true, "can't be blank"], match: [/^[a-zA-Z0-9]+$/, 'is invalid'], index: true},
email: {type: String, lowercase: true, unique: true, required: [true, "can't be blank"], match: [/\S+#\S+\.\S+/, 'is invalid'], index: true},
bio: String,
image: String,
following: [{ type: mongoose.Schema.Types.ObjectId, ref: 'User' }]
}, {timestamps: true});
So every User has array of users in an array in the key 'following'. I'm trying to output that list by first finding the user record through it's own id and then mapping through this array to find the followed users for this current user.
router.get('/users/friends', auth.required, function(req, res, next) {
var limit = 20;
var offset = 0;
if(typeof req.query.limit !== 'undefined'){
limit = req.query.limit;
}
if(typeof req.query.offset !== 'undefined'){
offset = req.query.offset;
}
User.findById(req.payload.id)
.then(function(user){
if (!user) { return res.sendStatus(401); }
return res.json({
users: user.following.map(function(username){
User.findById(username)
.then(function(userlist){
console.log('userlist:',userlist.username);
return userlist.username;
})
.catch(next)
})
})
})
.catch(next);
});
Now the console.log in this code outputs the correct data in the js console but I can't seem to find a way to deliver this to the client. So far my efforts bring forth 'null' values in the client. The correct amount of records but just null values. Any ideas how to fix this?
revised my code to this after taking advice below. Now it manages to get the first record to the client but then errors out with
UnhandledPromiseRejectionWarning: Unhandled promise rejection (rejection id: 1): Error: Can't set headers after they are sent.
Blockquote
router.get('/users/friends', auth.required, function(req, res, next) {
var limit = 20;
var offset = 0;
if (typeof req.query.limit !== 'undefined') {
limit = req.query.limit;
}
if (typeof req.query.offset !== 'undefined') {
offset = req.query.offset;
}
User.findById(req.payload.id)
.then(function(user) {
if (!user) {
return res.sendStatus(401);
}
Promise.all(
user.following
).then(function(userarray) {
console.log(userarray);
userarray.forEach(function(userid) {
Promise.all([
User.find({
_id: {
$in: userid
}
})
.limit(Number(limit))
.skip(Number(offset))
.populate('author')
.exec()
]).then(function(results) {
userdetails = results[0];
var userdetailsCount = results[1];
return res.json({
userdetails: userdetails.map(function(userdetail){
return userdetail;
})
});
})
})
})
})
.catch(next);
});

Answer 1

Your problem section is:
return res.json({
users: user.following.map(function(username){
User.findById(username)
.then(function(userlist){
console.log('userlist:',userlist.username);
return userlist.username;
})
.catch(next)
})
})
The bit User.findById(username) will return a promise. But you are not awaiting on that promise. I'm guessing you think that the then function following that promise, which logs userlist.username to the console and returns it, should mean that your map function returns a list of userlist.username's. But this is not the case. Your map function is returning an array of promises.
What you really want is a feature like Bluebird's Promise.map: http://bluebirdjs.com/docs/api/promise.map.html (or, look for a similar feature, to deal with arrays of promises, in whichever promise library you happen to be using).