aggregate聚合管道实例

db.order.insert([
  { order_id: "1", uid: 10, trade_no: "111", all_price: 100, all_num: 2 },
  { order_id: "2", uid: 7, trade_no: "222", all_price: 90, all_num: 2 },
  { order_id: "3", uid: 9, trade_no: "333", all_price: 20, all_num: 6 }
]);

db.order_item.insert([
  { order_id: "1", title: "商品鼠标1", price: 50, num: 1 },
  { order_id: "1", title: "商品鼠标2", price: 50, num: 1 },
  { order_id: "1", title: "商品鼠标3", price: 0, num: 1 },
  { order_id: "2", title: "牛奶", price: 50, num: 1 },
  { order_id: "2", title: "酸奶", price: 40, num: 1 },
  { order_id: "3", title: "矿泉水", price: 2, num: 5 },
  { order_id: "3", title: "毛巾", price: 10, num: 1 }
]);

db.order.aggregate([
    {
        $project:{
            order_id:1,
            uid:1,
            trade_no:1,
            all_price:1,
            all_num:1
        }
    },
    {
        $match:{
            all_price:{
                $gte:90
            }
        }
    },
    {
        $sort:{
            all_price:-1
        }
    },
    {
        $limit:2
    },
    {
        $skip:1
    }
])

db.order_item.aggregate([
    {
        $group:{
            _id:'$order_id',total:{
                $sum:'$num'
            }
        }
    },
])

db.order.aggregate([
    {
        $lookup:{
            from:'order_item',         // 要关联的表
            localField:'order_id',     //order表中的order_id
            foreignField:'order_id',   // order_item表中的order_id
            as:'items'
        }
    }    
])