db={"users":[{"_id":ObjectId("612e8361942c250f4ca280f3"),"_cart":[{"_id":"60e38a2f2f6338e00e4ceff6","count":2},{"_id":"60e38a2f2f6338e00e4ceff7","count":3},{"_id":"60e38a2f2f6338e00e4ceff8","count":4}],"name":"max1","email":"andrey.kupets1@i.ua"}],"products":[{"_id":ObjectId("60e38a2f2f6338e00e4ceff6"),"name":"Куб-лабиринт Svoora Замок","category":"Настольные игры","price":449.0},{"_id":ObjectId("60e38a2f2f6338e00e4ceff7"),"name":"Настольная игра Hasbro Классическая Монополия обновленная (C1009_121)","category":"Настольные игры","price":449.0},{"_id":ObjectId("60e38a2f2f6338e00e4ceff8"),"name":"Настольный футбол Merchant Ambassador 2-в-1","category":"Настольные игры","price":1899.0}]}

db.users.aggregate([{"$unwind":"$_cart"},{"$addFields":{"_cartObjId":{"$toObjectId":"$_cart._id"}}},{$lookup:{from:"products",localField:"_cartObjId",foreignField:"_id",as:"_productsInCart"}},{"$unwind":"$_productsInCart"},{"$group":{"_id":"$_id","_cart":{"$push":"$_cart"},"_productsInCart":{"$push":"$_productsInCart"},}}])