How to trigger a function whenever a mongoose document is updated - node.js

Question

I have a user model schema in mongoose which contains a list of friends and groups and stats info like so...
var user = new Schema({
email: { type: String, required: true, unique: true },
password: { type: String, required: true, select: false },
roles: [{ type: String, required: true }],
friends: [{ type: Schema.Types.ObjectId, ref: 'User' }],
groups: [{ type: Schema.Types.ObjectId, ref: 'Group' }],
stats : {
nbrFriends: { type: Number, required: false },
nbrGroups: { type: Number, required: false }
}
}, {
timestamps: true
});
I need to update the users stats whenever a change is made to the friends or groups fields to contain the new number of friends or groups etc. For example, when the following function is called on a user:
var addGroup = function(user, group, cb) {
user.groups.push(group);
User.findOneAndUpdate({ _id: user._id }, { $set: { groups: user.groups }}, { new: true }, function(err, savedResult) {
if(err) {
return cb(err);
}
console.log('updated user: ' + JSON.stringify(savedResult));
return cb(null, savedResult);
});
};
How could I make sure the stats is automatically updated to contain the new number of groups the user has? It seems like a middleware function would be the best approach here. I tried the following but this never seems to get called...
user.pre('save', function(next) {
var newStats = {
nbrGroups: this.groups.length,
nbrPatients: this.friends.length
};
this.stats = newStats;
this.save(function(err, result) {
if(err) {
console.log('error saving: ' + err);
} else {
console.log('saved');
}
next();
});
});

Answer 1

You need to use the middleware a.k.a. hooks:
Middleware (also called pre and post hooks) are functions which are passed control during execution of asynchronous functions.
See the docs:
http://mongoosejs.com/docs/middleware.html

Answer 2

From version 3.6, you can use change streams.
Like:
const Users = require('./models/users.js')
var filter = [{
$match: {
$and: [{
$or:[
{ "updateDescription.updatedFields.friends": { $exists: true } },
{ "updateDescription.updatedFields.groups": { $exists: true } },
]
{ operationType: "update" }]
}
}];
var options = { fullDocument: 'updateLookup' };
let userStream = Users.watch(filter,options)
userStream.on('change',next=>{
//Something useful!
})

Answer 3

You should update with vanilla JS and then save the document updated to trigger the pre-save hooks.
See Mongoose docs
If you have many keys to update you could loop through the keys in the body and update one by one.
const user = await User.findById(id);
Object.keys(req.body).forEach(key => {
user[key] = req.body[key];
}
const saved = await user.save();