Database
db={product:[{"_id":ObjectId("62d6fac11b6108a253a6478c"),"Name":"code","Category":"pic","Price":"2000","Description":"good one"},{"_id":ObjectId("62d6fad31b6108a253a6478d"),"Name":"Clickrf","Category":"picrdf","Price":"20002","Description":"good onef"},{"_id":ObjectId("62d6fae31b6108a253a6478e"),"Name":"Click3","Category":"pic3","Price":"2000.3","Description":"good one3"},{"_id":ObjectId("62d7fa7b54f0ecc72d85831c"),"Name":"Click","Category":"pic5","Price":"2000.2","Description":"good one5"}],cart:[{"_id":ObjectId("62d90f2ff3d39ad716146cae"),"user":ObjectId("62d7a948d05082b22306ff73"),"products":[{"item":ObjectId("62d6fac11b6108a253a6478c"),"quantity":3},{"item":ObjectId("62d6fae31b6108a253a6478e"),"quantity":2},{"item":ObjectId("62d6fad31b6108a253a6478d"),"quantity":1},{"item":ObjectId("62d7fa7b54f0ecc72d85831c"),"quantity":1}]}]}
Query
db.cart.aggregate([{$match:{user:ObjectId("62d7a948d05082b22306ff73")}},{$unwind:"$products"},{$project:{item:"$products.item",quantity:"$products.quantity"}},{$lookup:{from:"product",localField:"item",foreignField:"_id",as:"product"}}])