Aggregate Query Mongodb for Rating System - node.js

Question

I have 2 collections one is user and other is college. I want a rating schema with aggregate query user 1 gives 2 rating to college 1
I made a rating schema like
rate = mongoose.Schema({
rating: number,
userId:
collegeid:})
college comes on the basis of score user gets. And then user is allowed to rate the colleges, he has been shown.
So how to write the query to fetch the rating. How can I achieve this?

Answer 1

Let's assume the rate collection has data like this:
var r = [
{ rating: 1, userId: "U1", collegeid: "C1" },
{ rating: 1, userId: "U1", collegeid: "C2" },
{ rating: 3, userId: "U1", collegeid: "C3" },
{ rating: 2, userId: "U2", collegeid: "C1" },
{ rating: 6, userId: "U2", collegeid: "C2" },
{ rating: 7, userId: "U3", collegeid: "C1" },
{ rating: 5, userId: "U3", collegeid: "C4" },
{ rating: 3, userId: "U3", collegeid: "C3" }
];
Here are some useful queries to get started. In general, aggregate() is The New find() (since at least v3.2) and although slightly more complicated for the simplest expressions, it is vastly more powerful and it should be your starting point.
// Lookup all ratings available for collegeid C1:
c=db.rate.aggregate([
{$match: {"collegeid":"C1"}}
]);
// Lookup all ratings by user U3:
c=db.rate.aggregate([
{$match: {"userId":"U3"}}
]);
// Lookup all ratings by user U3 for college C1:
c=db.rate.aggregate([
{$match: {"userId":"U3","collegeid":"C1"}}
]);
// Lookup all ratings by user U3 OR college C1:
c=db.rate.aggregate([
{$match: {$or: [
{"userId":"U3"},
{"collegeid":"C1"}
]}}
]);
// Get counts, avg, max & min ratings for all colleges:
c=db.rate.aggregate([
{$group: {_id:"$collegeid", n:{$sum:1},
avg:{$avg:"$rating"},
max:{$max:"$rating"},
min:{$min:"$rating"}}}
]);
// Who rated colleges the highest?
c=db.rate.aggregate([
{$sort: {"rating":-1}}, // sort descending
// Use $first operator to capture only the first item for each unique collegeid
// in the sorted material flowing into this stage:
{$group: {_id:"$collegeid", who:{$first:"$userId"}, rating:{$first:"$rating"} }}
]);

Answer 2

You can use the built-in mongoose populate function if you define your Schema correctly.
Please check the link below
https://mongoosejs.com/docs/populate.html
The "mongoose" way would look something like
ratingSchema = mongoose.Schema({
rating: Number,
userId: {
ref: 'users', // Name of the collection that you would like to link to
type: mongoose.SchemaTypes.ObjectId // I Just added the object ID type you could use
// a string or whatever your id is
},
collegeId: {
ref: 'colleges', // Name of the collection that you would like to link to
type: mongoose.SchemaTypes.ObjectId
},
})
const Rating = mongoose.model('rating', ratingSchema);
And then you can use
Rating
.findOne({})
.populate('userId')
.populate('collegeId');
It is good however to read up and learn about the aggregate pipeline, but mongoose can do a lot of heavy lifting for you.