Fetch grouped items from database - node.js

Question

I'm trying to group values together in mongoose. I have a "Review" schema with the following fields:
{ userId, rating, comment }
There are many documents with the same userId. How can I retrieve them in the following format:
{userId: [...allRatings]
Or even better, is there a way to retrieve the averages for each userId? so like this: {userId: 2.8}
I know it's possible and very simple to do in node.js, but is there a way of doing it with mongoose?

Answer 1

Mongoose is really just a vehicle to pass commands to your mondoDB server, so accomplishing what you want in mongoose isn't dissimilar to accomplishing it in the mongo shell.
Here is the aggregation you're looking for:
db.collection.aggregate([
{
"$group": {
"_id": "$userId",
"ratings": {
$push: "$rating"
}
}
},
{
"$project": {
"_id": false,
"userId": "$_id",
"avgRating": {
"$avg": "$ratings"
}
}
}
])
The first stage of the pipeline groups all ratings by useId. The second stage calculates the ratings average and pretties up the key display. That's it. The result will be this:
[
{
"avgRating": 2.8,
"userId": 110
},
{
"avgRating": 3.275,
"userId": 100
}
]
Here is a playground for you: https://mongoplayground.net/p/yXVxk4klabB
As for how to specifically run this command in mongoose, well that's pretty straightforward:
const YourModel = mongoose.model('your_model');
...
YourModel.aggregate([
{
"$group": {
"_id": "$userId",
"ratings": {
$push: "$rating"
}
}
},
{
"$project": {
"_id": false,
"userId": "$_id",
"avgRating": {
"$avg": "$ratings"
}
}
}
])
.then(result => {
console.log(result);
})