Mongoose - REST API - Schema With Query to different model - node.js

Question

I'm trying to avoid DB Callback Queries.
Assuming that you have two schemas that looks like so :
1st) User Schema
username : {type: String, unique: true},
age : {type: Number}
2nd) Activity Schema
owner: [{type: Schema.Types.ObjectId, ref: 'User'}],
city: {type: String},
date: {type: Date}
So far so good.
Now lets say you have a route to /user/:id, what you would expect is to get the username and the age, but what if I would also like to return on that route the latest activity?
EDIT: Please note that latest activity isn't a value in the database. it's calculated automatically like activity.find({owner: ObjectId(id)}).sort({date: -1}).limit(1)
What is done right now:
User.findOne({username:req.params.username}).lean().exec(function(err,userDoc)
{
if(err) return errHandler(err);
Activity.findOne({owner:userDoc.username}).sort({date:-1}).exec(function(err,EventDoc){
if(err) return errHandler(err);
userDoc.latest_activity = EventDoc._id;
res.json(userDoc);
res.end();
})
})
The problem with the snippet above is that it is hard to maintain,
What if we want to add more to this API functionality? We would end in a callback of hell of queries unless we implement Q.
We tried to look at Virtual but the issue with that is that you can't
really query inside a mongoose Virtual, since it returns a
race-condition, and you are most likely not get that document on time.
We also tried to look at populate, but we couldn't make it since the documentation on populate is super poor.
QUESTION:
Is there anyway making this more modular?
Is there any way avoiding the DB Query Callback of Hell?
For example is this sort of thing possible?
User.findOne({username:req.params.username}).lean().populate(
{path:'Event',sort:{Date:-1}, limit(1)}
).exec(function(req,res))...
Thanks!

Answer 1

In this case, the best way to handle it would be to add a post save hook to your Activity schema to store the most recent _id in the latest_activity path of your User schema. That way you'd always have access to the id without having to do the extra query.
ActivitySchema.post('save', function(doc) {
UserSchema.findOne({username: doc.owner}).exec(function(err, user){
if (err)
console.log(err); //do something with the error
else if (user) {
user.latest_activity = doc._id;
user.save(function(err) {
if (err)
console.log(err); //do something with the error
});
}
});
});

Answer 2

Inspired by #BrianShambien's answer you could go with the post save, but instead of just storing the _id on the user you store a sub doc of only the last activity. Then when you grab that user it has the last activity right there.
User Model
username : {type: String, unique: true},
age : {type: Number},
last_activity: ActivitySchema
Then you do a post save hook on your ActivitySchema
ActivitySchema.post('save', function(doc) {
UserSchema.findOne({username: doc.owner}).exec(function(err, user){
if (err) errHandler(err);
user.last_activity = doc;
user.save(function(err) {
if (err) errHandler(err);
});
});
});
**********UPDATE************
This is to include the update to the user if they are not an owner, but a particpant of the the activity.
ActivitySchema.post('save', function(doc) {
findAndUpdateUser(doc.owner, doc);
if (doc.participants) {
for (var i in doc.participants) {
findAndUpdateUser(doc.participants[i], doc);
}
}
});
var findAndUpdateUser = function (username, doc) {
UserSchema.findOne({username: username}).exec(function (err, user) {
if (err) errHandler(err);
user.last_activity = doc;
user.save(function (err) {
if (err) errHandler(err);
});
});
});