node.js - MongoDB : How to multiply a field that appears only in $project?

Question

我通过以下方式加入($lookup)两个集合:

  const LEAD_PRICE  = ....; // doesn't matter 
  Client.aggregate(
      [
        {
          $lookup: {
            from: "clientboughtleads",
            localField: "_id",
            foreignField: "Client",
            as: "ClientLeads"
          }
        },

        {
          $project: {
            ClientId: "$_id",
            RegistrationDate: "$date",
            TotalPurchasedLeads: {
              $size: "$ClientLeads"
            },
            IncomeFromClient: {     // This one is always undefined
              $multiply: [LEAD_PRICE / 2, "$TotalPurchasedLeads"]
            }
          }
        }
      ]

我想通过 const 和已计算的字段 - TotalPurchasedLeads 的乘数来计算 IncomeFromClient ，但它返回 NULL :

 [
[0]   {
[0]     _id: 5e0e43ae82a71e0017dccb20,
[0]     ClientId: 5e0e43ae82a71e0017dccb20,
[0]     RegistrationDate: 2020-01-02T21:25:34.000Z,
[0]     TotalPurchasedLeads: 4,
[0]     IncomeFromClient: null      // This one is not calculated 
[0]   }
[0] ]

在计算 $projected 字段时如何获取 $lookup 集合的大小？

Answer 1

我们可以在项目中使用 key 后，请使用 addFields 关键字。

 const LEAD_PRICE  = ....; // doesn't matter 
  Client.aggregate(
      [
        {
          $lookup: {
            from: "clientboughtleads",
            localField: "_id",
            foreignField: "Client",
            as: "ClientLeads"
          }
        },
   {
     $addFields: {
       TotalPurchasedLeads: {
              $size: "$ClientLeads"
          }
     }
   },
        {
          $project: {
            ClientId: "$_id",
            RegistrationDate: "$date",
            TotalPurchasedLeads:1,
            IncomeFromClient: {     // This one is always undefined
              $multiply: [LEAD_PRICE / 2, "$TotalPurchasedLeads"]
            }
          }
        }
      ]