[{_id:"6138e5866b19971ba8a60851",tasks:[{_id:"6138e5ad6b19971ba8a60852",taskName:"Hello"},{_id:"6138e5ad6b19971ba8a543252",taskName:"Hello 2"}]},/** document 2*/{_id:"6138e5866b19971ba8a76851",tasks:[{_id:"6138e5ad6f19971ba8a60852",taskName:"World"},{_id:"6138e5ad6sd19971ba8a543252",taskName:"World 2"}]}]

db.collection.aggregate([{"$unwind":"$tasks"},{"$match":{"tasks._id":{"$in":["6138e5ad6b19971ba8a60852","6138e5ad6b19971b568a60852"]}}},{"$group":{"_id":null,"tasks":{"$push":"$tasks"}}},{"$project":{"_id":0}}])