Calculating average value per document with sort - node.js

Question

I have a mongoose model defined as such:
freelancerSchema = mongoose.Schema({
_id: { type: String, default: shortid.generate},
fname: String,
lname: String;
ratings: [{
rating: Number,
employer: {
type: String,
ref: 'Employer'
}
}],
...
}]
This schema represents a mongoose model for a Freelancer collection. My question is: in a certain query I need to find all freelancers with all their data and calculate the average rating for each of them. In the end, I would get an array of freelancers, each having their own calculated average rating preferably stored in a new field "avg_rating" or something like that.
I've tried looking into the mongodb Aggregate but I honestly didn't understand much.
Thanks in advance and sorry if my explanation wasn't precise enough.

Answer 1

If we are going to play code golf here, then the expression can be shortened:
Freelancer.aggregate([
{ "$addFields": {
"rating_avg": {
"$reduce": {
"input": "$ratings",
"initialValue": 0,
"in": {
"$add": [
"$$value",
{ "$divide": [ "$$this.rating", { "$size": "$ratings" } ] }
]
}
}
}
}},
{ "$sort": { "rating_avg": -1 } }
],function(err, results) {
res.send(results)
})
Or even a bit shorter using $avg and $map:
Freelancer.aggregate([
{ "$addFields": {
"rating_avg": {
"$avg": {
"$map": {
"input": "$ratings",
"as": "el",
"in": "$$el.rating"
}
}
}
}},
{ "$sort": { "rating_avg": -1 } }
],function(err, results) {
res.send(results)
})
And of course the shortest yet, being allowed since MongoDB 3.2 (modifying with $project of course):
Freelancer.aggregate([
{ "$addFields": {
"rating_avg": { "$avg": "$ratings.rating" }
}},
{ "$sort": { "rating_avg": -1 } }
],function(err, results) {
res.send(results)
})
All also using $addFields as an alternate to $project when using MongoDB 3.4, which is where $reduce becomes available. The second form when modified with $project also becomes valid for MongoDB 3.2, as is also true ( and noted ) of the third.

Answer 2

After messing around with my code and reading some other stacks, I found a solution that works fine for my needs:
Freelancer.aggregate(
[{
$project: {
fname: "$fname",
lname: "$lname",
rating_avg: {
$divide: [{
$reduce: {
input: "$ratings.rating",
initialValue: 0,
in: {
$sum: ["$$value", "$$this"]
}
}
}, {
$size: "$ratings"
}]
}
}
},
{
$sort: {
rating_avg: -1
}
}
],
function (err, results) {
res.send(results);
});
});
Hope this can help somebody else in the future.