db={"add_to_cart":[{add_to_cart_products:[{product:ObjectId("5f059f8e0b4f3a5c41c6f54d"),product_quantity:5,product_item:ObjectId("5f4dddaf8596c12de258df20")}],add_to_cart_product_total:5,add_to_cart_discount:50}],"shop_db_products":[{_id:ObjectId("5f059f8e0b4f3a5c41c6f54d"),product_name:"La Gioiosa Prosecco",product_description:"No Description",product_overview:"No Overview",product_items:[{product_item_number:"781239007465",product_price:14.99,product_images:["pro03655-781239007465-1.png"],product_item_is_active:true,_id:ObjectId("5f4dddaf8596c12de258f021")},{product_item_number:"850651005110",product_price:12.99,product_images:["default.png"],product_item_is_active:true,_id:ObjectId("5f4dddaf8596c12de258df20")}],product_created_date:ISODate("2020-07-08T10:29:05.892Z"),product_status_is_active:true}]}

db.add_to_cart.aggregate([{$unwind:"$add_to_cart_products"},{$lookup:{from:"shop_db_products",let:{productId:"$add_to_cart_products.product",purchaseQuantity:"$add_to_cart_products.product_quantity",product_item_id:"$add_to_cart_products.product_item"},pipeline:[{$match:{$expr:{$eq:["$_id","$$productId"]}}},{$project:{product_name:1,product_id:"$_id",product_purchase_quantity:"$$purchaseQuantity",product_item:{$reduce:{input:{$filter:{input:"$product_items",cond:{$eq:["$$product_item_id","$$this._id"]}}},initialValue:{},in:{product_price:"$$this.product_price",product_images:"$$this.product_images"}}}}}],as:"add_to_cart_products"}},{$unwind:"$add_to_cart_products"},{$group:{_id:"$_id",add_to_cart_discount:{$first:"$add_to_cart_discount",},add_to_cart_product_total:{$first:"$add_to_cart_product_total",},add_to_cart_products:{$push:"$add_to_cart_products"}}}])