mongo aggregation:- push all fields using Spring - spring

Question

if I have a collection of books :-
{author: "tolstoy", title:"war & peace", price:100, pages:800}
{author: "tolstoy", title:"Ivan Ilyich", price:50, pages:100}
and if i want a result like this after grouping them by author :-
{ author: "tolstoy",
books: [
{author: "tolstoy", title:"war & peace", price:100, pages:800}
{author: "tolstoy", title:"Ivan Ilyich", price:50, pages:100}
]
}
using raw mongo queries I can do something like this:-
{$group: {
_id: "$author",
books:{$push: {author:"$author", title:"$title", price:"$price", pages:"$pages"}},
}}
But how do I do this using spring , I tried something like this:-
private GroupOperation getGroupOperation() {
return group("author").push("title").as("title").push("price").as("price").push("pages").as("pages");
}
but this does not seem to work. Any help would be appreciated.
UPDATE:-
I used the solution as in the link suggested by #Veeram and it works great but I ran into another issue when I project it. I have my projection class which looks like:-
public class BookSummary{
private String author;
private List<Book> bookList;
//all getters and setters below
}
The group method looks like this:-
private GroupOperation getGroupOperation() {
return group("author").push(new BasicDBObject("id","$_id").append("title","$title").append("pages","$pages").append("price","$price")).as("bookList");
}
the projection method looks like this:-
private ProjectionOperation getProjectOperation() {
return project("author").and("bookList").as("bookList");
}
and the final aggregation operation:-
mongoTemplate.aggregate(Aggregation.newAggregation(groupOperation,projectionOperation), Book.class, BookSummary.class).getMappedResults();
However this gives the result:-
[
{
"author": null,
"bookList": [
{
"id": null,
"title": "title1",
"pages": "100",
"price":"some price"
},
{
"id": null,
"title": "title2",
"pages": "200",
"price":"some price"
}
]
}
]
Why is the author and id null here? Any help would be appreciated

Answer 1

You should be projecting using _id instead in the project phase.
private ProjectionOperation getProjectOperation() {
return project("_id").and("bookList").as("bookList");
}