db={"user":[{_id:"61005f388308a717883ad3d2","productLine":{"product":[],"created_count":4,"purchased":[],"purchased_count":0,},"following":{"users":[],"count":0},"followers":{"users":[],"count":0},"product_picture":"xxxxxxxxx","__v":0}],"product":[{"_id":"5fefa2556308ab102854baf7"},{"_id":"5fefa2556308ab102854baf8"},{"_id":"5fefa2556308ab102854baf9"},{"_id":"5fefa2556308ab102854bafc"},{"_id":"5fefa2556308ab102854bag7"},{"_id":"5fefa2556308ab102854bah8"},{"_id":"5fefa2556308ab102854bac5"},{"_id":"5fefa2556308ab102854babc"}]}

db.user.aggregate([{"$lookup":{"from":"product","pipeline":[{"$project":{_id:1}},{"$sample":{"size":4}}],"as":"productLookup"}},{"$addFields":{"productLine.product":{"$map":{"input":"$productLookup","as":"p","in":"$$p._id"}}}},{"$project":{productLookup:false}}])