MongoDB常用語法集合

一對多關聯查詢（商品表爲主表、取商品規格集合）：

db.mall_goods.aggregate([
   {
     $lookup:
       {
         from: "mall_specs",
         localField: "id",
         foreignField: "goods_id",
         as: "inventory_docs"
       }
  }
]);

一對多關聯查詢（訂單表爲主表，根據商品ID取商品名稱、商品規格集合）：

db.mall_order.aggregate([
  {
    $lookup:
      {
        from: "mall_goods",
        localField: "id",
        foreignField: "goods_id",
        as: "goods"
      }
 },
  {
    $lookup:
      {
        from: "mall_specs",
        localField: "goods_id",
        foreignField: "goods_id",
        as: "specs"
      }
 }
])