[{"id":1,"name":"N1","tasks":{"project1":{"task7":{"count":10,"time":1.2},"task3":{"count":56,"time":1050}},"project25":{"task7":{"count":25,"time":10}}}}]

db.collection.aggregate([{$project:{_id:0,tasks:{$objectToArray:"$tasks"}}},{$project:{tasks:{$arrayToObject:{$map:{input:"$tasks",in:{"k":"$$this.k","v":{task7:{count:"$$this.v.task7.count"}}}}}}}}])