Mongoose/Mongodb Aggregate - group and average multiple fields - mongodb

Question

I have a Post model with 2 fields : date and rating. How would I go about getting an average aggregate rating for each date? So group by date first and then average the rating across all posts for that date. I need to do this within mongoose but their docs are so difficult to understand.
const PostSchema = new Schema({
date: {
type: String,
default: getToday() //this is just a new Date() formatted
},
rating: {
type: Number,
required: true
}
},
)
This gives me the average across all dates but I can't figure out how to filter it by date:
Post.aggregate([
{ $group: { _id: null, avgRating: { $avg: '$rating' }}}
])
.then(function (res) {
console.log(res[0]["avgRating"]);
})

Answer 1

This worked for me:
Post.aggregate([
{ $group: { _id: "$date", avgRating: { $avg: '$rating' }}}
]).
then(function (res) {
console.log(res);
})
Output:
[
{ _id: 'Aug 18, 2021', avgRating: 3.0212234706616727 },
{ _id: 'Aug 19, 2021', avgRating: 2.9680319680319682 },
{ _id: 'Aug 20, 2021', avgRating: 3.023976023976024 },
{ _id: 'Aug 17, 2021', avgRating: 2.9600665557404326 },
{ _id: 'Aug 21, 2021', avgRating: 3.072661217075386 }
]
BUT it would be great if I could somehow filter this based on other factors. For example, each post has an author (reference to User model). How would I go about filtering based on the author's country.name or gender?
User model:
const userSchema = new Schema({
email: {
type: String,
required: true,
unique: true
},
birthday: {
type: Date,
required: true,
},
gender:{
type: String,
required: true
},
country:{
name: {
type: String,
required: true
},
flag: {
type: String,
// default: "/images/flags/US.png"
}
},
avatar: AvatarSchema,
displayName: String,
bio: String,
coverColor: {
type: String,
default: "#343a40"
},
posts: [
{
type: Schema.Types.ObjectId,
ref: "Post"
}
],
comments: [
{
type: Schema.Types.ObjectId,
ref: "Comment"
}
],
postedToday: {
type: Boolean,
default: false
},
todaysPost: {
type: String
}
})
Something like this
Post.aggregate([
{$match: {"date": today}},
{$group: {_id: {"country": "$author.country.name"}, avgRating: {$avg: "$rating"}}}
]).then(function(res) {
console.log(res)
})