db={"db_task":[{"_id":ObjectId("5d8b522d0cf2579c57bc8ce0"),"users":[{"user_id":ObjectId("5d8b522d0cf2579e27bc8ce3"),"is_finished":false},{"user_id":ObjectId("5d6f6d25e079b9fb7d858236"),"is_finished":false}]}],"db_user":[{"_id":ObjectId("5d8b522d0cf2579e27bc8ce3"),"first_name":"Harry","last_name":"Paul"},{"_id":ObjectId("5d6f6d25e079b9fb7d858236"),"first_name":"Aaron","last_name":"Potter"}]}

db.db_task.aggregate([{$unwind:"$users"},{$lookup:{from:"db_user",localField:"users.user_id",foreignField:"_id",as:"user_info"}},{$project:{users:{user_id:1,is_finished:1,user_info:"$user_info"}}},{$group:{_id:"$_id",users:{$push:"$users"}}},{$project:{"users.user_info._id":0}}])