Mongo DB aggregation to convert books to array of authors - mongodb

Question

My book collection looks like this:
{
"book": {
"title": "Book1",
"author": "Tom"
},
"quantity": 3
},
{
"book": {
"title": "Book2",
"author": "Tom"
},
"quantity": 4
},
{
"book": {
"title": "Book3",
"author": "Dick"
},
"quantity": 9
},
{
"book": {
"title": "Book4",
"author": "Harry"
},
"quantity": 6
},
{
"book": {
"title": "Book5",
"author": "Chris"
},
"quantity": 7
},
{
"book": {
"title": "Book6",
"author": "Dick"
},
"quantity": 10
}
This collection has book documents with title, author and quantity.
I want help in coming up with aggregation which would output the array of authors with their books and quantity. Example - Tom has authored "Book1" and "Book2", "Dick" has authored "Book6" and "Book3".
authors: [
{
"name": "Tom",
"books": [
{
"title": "Book1",
"quantity": "3"
},
{
"title": "Book2",
"quantity": "4"
}
]
},
{
"name": "Dick",
"books": [
{
"title": "Book6",
"quantity": "10"
},
{
"title": "Book3",
"quantity": "9"
}
]
},
{
"name": "Harry",
"books": [
{
"title": "Book4",
"quantity": "6"
}
]
}
]

Answer 1

Try this aggregation:
db.collection.aggregate([{
$group: {
_id: "$book.author",
books: {
$push: {
title: "$book.title",
quantity: "$quantity"
}
}
}},{
$project: {
"name": "$_id",
"books": 1,
"_id": 0
}}])
I tried it on mongo playground:
https://mongoplayground.net/p/m-QbX-uzkkt

Answer 2

Try as below:
db.collection.aggregate([
{
$group: {
_id: "$book.author",
books: { $push: { "title": "$book.title", "quantity": "$quantity"} }
},
}
])
Output will be
/* 1 */
{
"_id" : "Chris",
"books" : [
{
"title" : "Book5",
"quantity" : 7
}
]
},
/* 2 */
{
"_id" : "Harry",
"books" : [
{
"title" : "Book4",
"quantity" : 6
}
]
},
/* 3 */
{
"_id" : "Dick",
"books" : [
{
"title" : "Book3",
"quantity" : 9
},
{
"title" : "Book6",
"quantity" : 10
}
]
},
/* 4 */
{
"_id" : "Tom",
"books" : [
{
"title" : "Book1",
"quantity" : 3
},
{
"title" : "Book2",
"quantity" : 4
}
]
}