db={"books":[{_id:ObjectId("625efa44f1ba751c8275ea51"),bookName:"test",contributors:[ObjectId("625efa44f1ba751c8275ea52"),ObjectId("625efa44f1ba751c8275ea53")]/**other fields*/}],"users":[{_id:ObjectId("625efa44f1ba751c8275ea52"),first_name:"Luigi"/**many other fields*/},{_id:ObjectId("625efa44f1ba751c8275ea53"),first_name:"Mario"/**many other fields*/}]}

db.books.aggregate([{"$lookup":{"from":"users","localField":"contributors","foreignField":"_id","as":"contributors"}},{"$unwind":"$contributors"},{"$group":{"_id":"_id",bookName:{$first:"$bookName"},"contributors":{"$push":"$contributors"}}}])