db={books:[{"name":"Harry Potter","author":"1"},{"name":"Lord of The Rings"}],authors:[{_id:"1",name:"J.K.Rowling"}]}

db.books.aggregate([{$lookup:{from:"authors",localField:"author",foreignField:"_id",as:"bookAuthor"}},{$unwind:"$bookAuthor"},{$project:{name:1,bookAuthor:{name:1}}}])