实战案例

// 商品
db.products.insertMany([
    { _id: 1, name: "苹果", price: 5.5, stock: 100 },
    { _id: 2, name: "香蕉", price: 3.2, stock: 200 },
    { _id: 3, name: "橘子", price: 6.0, stock: 80 }
]);

// 购物车（嵌套数组）
db.carts.insertOne({
    user_id: 1001,
    items: [
        { product_id: 1, qty: 2 },
        { product_id: 3, qty: 1 }
    ]
});

// 加购：不存在就建，存在就加数量
db.carts.updateOne(
    { user_id: 1001, "items.product_id": 2 },
    { $inc: { "items.$.qty": 1 } }
);
db.carts.updateOne(
    { user_id: 1001, "items.product_id": { $ne: 2 } },
    { $push: { items: { product_id: 2, qty: 1 } } }
);

// 算购物车总额（聚合 $lookup 关联商品）
db.carts.aggregate([
    { $match: { user_id: 1001 } },
    { $unwind: "$items" },
    { $lookup: {
        from: "products",
        localField: "items.product_id",
        foreignField: "_id",
        as: "product"
    } },
    { $project: {
        subtotal: { $multiply: ["$items.qty", { $arrayElemAt: ["$product.price", 0] }] }
    } },
    { $group: { _id: null, total: { $sum: "$subtotal" } } }
]);