db={product:[{"_id":"54759eb3c090d83494e2d804","product_name":"sample product","image":"default.png","price":55,"discount":5,}],order:[{"user_name":"sample1","product_list":[{"product_id":"54759eb3c090d83494e2d804","quantity":5}]}]}

db.order.aggregate([{$unwind:"$product_list"},{$lookup:{from:"product",as:"product_list.product_id",let:{product_id:"$product_list.product_id"},pipeline:[{$match:{$expr:{$eq:["$$product_id","$_id"]}}},{$project:{_id:0,product_name:1}}]}},{$unwind:{path:"$product_list.product_id"}},{$group:{_id:"$_id",user_name:{$first:"$user_name"},product_list:{$push:"$product_list"}}}])