MongoDB 在 3 个集合之间聚合和查找

Question

我正在尝试创建一个简单的客户/产品/订单数据库。 This is the customer collection

This is the order collection

This is the product collection

我更喜欢使用 ref 键而不是嵌入文档只是为了锻炼。 我正在尝试按客户名称进行分组，然后为每个客户求和他的总订单数 = Sum (Product_price x Quantity)。

这是我尝试过的查询，但它不起作用：/

db.customers.aggregate([
  {$lookup:{from:"orders", localField:"ordered",foreignField:"_id", as:"Bought"}},
  {$lookup:{from:"products", localField:"Bought.Includes.product",foreignField:"_id", as:"User_products"}},
  {$group:{_id:{name:"$name"},total: { $multiply: [ "$price", "$quantity" ] }}},
  {$project: {name:1, total:1}  }
]).pretty()

Answer 1

你几乎明白了，只是一些问题。代码注释中的解释

[
  {
    $lookup: {
      from: "orders",
      localField: "ordered",
      foreignField: "_id",
      as: "Bought"
    }
  },
  {
    $unwind: '$Bought' //  You have to use $unwind on an array if you want to use a field in the subdocument array to further usage with `$lookup` 
  },
  {
    $unwind: '$Bought.Includes' // Also $unwind here
  },
  {
    $lookup: {
      from: "products",
      localField: "Bought.Includes.product",
      foreignField: "_id",
      as: "User_products"
    }
  },
  {
    $unwind: '$User_products' // also $unwind here
  },
  {
    $group: {
      _id: "$name" , // [optional] only one field to use as key, no need to wrap in an object
      total: {
        $sum: { // don't forget to $sum here
          $multiply: [
            "$User_products.Price", // access the value with the full object path
            "$Bought.Includes.quantity" // access the value with the full object
          ]
        } 
      }
    }
  },
]

更多关于$unwind

提示：确保大写/小写的用法一致，我注意到你使用product vs Price vs Quantity 会更容易出现拼写错误