[{"Id":"Id1","Title":"Title1","User":[{"Id":"Userid2","Title":"User Title1","Concerts":[{"Id":"Concert4","Title":"Concert title1","Users":{"Id":"Userid2","Title":"User Title2 Concert4"}},{"Id":"Concert1","Title":"Concert title1","Users":{"Id":"Userid2","Title":"User Title2 Concert1"}}]},{"Id":"Userid1","Title":"User Title1","Concerts":[{"Id":"Concert1","Title":"Concert title1","Users":{"Id":"Userid1","Title":"User Title Concert1"}},{"Id":"Concert2","Title":"Concert title1","Users":{"Id":"Userid1","Title":"User Title Concert2"}},{"Id":"Concert3","Title":"Concert title1","Users":{"Id":"Userid1","Title":"User Title Concert3"}}]}]}]

db.collection.aggregate([{$set:{User:{$map:{input:"$User",as:"user",in:{$mergeObjects:["$$user",{Concerts:{$map:{input:"$$user.Concerts",as:"concert",in:{$mergeObjects:["$$concert",{UserTitle:"$$concert.Users.Title"}]}}}}]}}}}}])