[{_id:1,tasks:[{_id:1,assigned:[{_id:1,name:"assigned1",solutions:[{_id:1,name:"solution 1"},{_id:2,name:"solution 2"}]},{_id:2,name:"assigned2",solutions:[{_id:1,name:"solution 1"},{_id:2,name:"solution 2"}]}]}]}]

db.collection.aggregate([{$set:{tasks:{$map:{input:"$tasks",as:"task",in:{$mergeObjects:["$$task",{assigned:{$map:{input:"$$task.assigned",as:"assigned",in:{$mergeObjects:["$$assigned",{solutions:{$map:{input:"$$assigned.solutions",as:"solution",in:{$mergeObjects:["$$solution",{taskName:"$$assigned.name"}]}}}}]}}}}]}}}}}])