javascript - MongoDB:如何获取特定文档的计数？

Question

我有一个用户模式:

const signUpTemplate = new mongoose.Schema({
  fullname: {
    type: String,
  },
  purchasedCourses: {
    type: Array,
    default: [] //Here courseIdList is pushed with unique ID and course name
  }
});

以及类(class)架构:

const courseIdList = new mongoose.Schema({
  _id: { type: String },
  courseName: { type: String },
  purchaseDate: {
    type: Date,
    default: Date.now,
  },
});

我怎样才能得到拥有相同类(class)的用户总数？例如，如果 10 个不同的用户购买了类(class)名称“A”，如何获得这个总数？

Answer 1

使用$lookup，您可以在users 集合中查找courses 集合的匹配记录。

db.courses.aggregate(
   [{ $lookup: { 
      from: "users", 
      localField: "_id", 
      foreignField: "purchasedCourses._id", 
      as: "coursesCount" 
   }}, 
   { $addFields: { "coursesCount": { $size: "$coursesCount" } } }]
)

Working example

Read More在 $lookup;

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