[{_id:1,user:"1234",stop:{title:"book1",author:"xyz",page:32}},{_id:2,user:"7890",stop:[{title:"book2",author:"abc",page:5},{title:"book3",author:"ijk",page:100}]}]

db.collection.aggregate([{$project:{"stop.title":1,document:"$$ROOT",}},{$unset:["document.stop"]}])