db={"Books":[{"_id":8751,"title":"The Banquet","author":"AB","copies":2,"publisher":[1,2]},{"_id":8752,"title":"Divine Comedy","author":"Dante","copies":1,"publisher":[3,1]},],"Publisher":[{id:1,name:"p 1"},{id:2,name:"p 2"},{id:3,name:"p 3"},{id:4,name:"p 4"}]}

db.Books.aggregate([{"$lookup":{"from":"Publisher","localField":"publisher","foreignField":"id","as":"publishers"}},{"$unwind":"$publishers"},{"$group":{"_id":"$publishers.id","publisher":{"$first":"$publishers.name"}}},{"$project":{"_id":0,"id":"$_id","publisher":1}},{"$sort":{"id":1}}])