mongodb aggregate group by category - mongodb

Question

I am new in using MongoDB's aggregation framework and here I have below a schema:
var bookSoldSchema = new Schema({
buyer:{
type: Number
}
book: {
type: Schema.Types.ObjectId, ref: 'Book'
}
});
var bookSchema = new Schema({
bookName: { type: String },
categories: [{ type: Schema.Types.ObjectId, ref: 'BookCategory'}],
})
Each book has multiple categories, I would like to show the top 5 best seller categories and each category I need show top 3 books which where sold most, and I also need show the category name, the results which I need is:
[
{
categoryId: xxx,
categoryName:xxx,
top3books:[
{bookId:xxx, bookName:xxx},
{bookId:xxx, bookName:xxx},
{bookId:xxx, bookName:xxx},
]
},
{
categoryId: xxx,
categoryName:xxx,
top3books:[
{bookId:xxx, bookName:xxx},
{bookId:xxx, bookName:xxx},
{bookId:xxx, bookName:xxx},
]
}
]
How can I go about this in MongoDB?

Answer 1

You can run the following aggregation pipeline which assumes you have a model BookSold that uses the bookSoldSchema above, books and bookcategories underlying collections:
BookSold.aggregate([
{
"$lookup": {
"from": "books",
"localField": "book",
"foreignField": "_id",
"as": "book_join"
}
},
{ "$unwind": "$book_join" },
{ "$unwind": "$book_join.categories" },
{
"$lookup": {
"from": "bookcategories",
"localField": "book_join.categories",
"foreignField": "_id",
"as": "categories"
}
},
{ "$unwind": "$categories" },
{ "$sort": { "categories.name": 1, "buyer": -1 } },
{
"$group": {
"_id": "$categories.name",
"categoryId": { "$first": "$categories._id" },
"buyer": { "$first": "$buyer" },
"books": {
"$push": {
"bookId": "$book_join._id",
"bookName": "$book_join.bookName"
}
}
}
},
{ "$sort": { "buyer": -1 } },
{ "$limit": 5 },
{
"$project": {
"_id": 0,
"categoryId": 1,
"categoryName": "$_id",
"top3books": { "$slice": [ "$books", 3 ] }
}
}
], function(err, result) {
if (err) handleError(err);
console.log(JSON.stringify(result, null, 4));
})