Mongoose聚合返回不需要的空数组

Question

我有两个模型；一个用于用户，另一个用于学习组。每个StudyGroup都有一个唯一的GUID字段。用户模型有一个studyGroups字段，它是字符串GUID的数组。一个用户可以加入多个学习组。

用户模型

 const userSchema = new Schema({
  cuid: { type: 'String', required: true },
  firstName: { type: 'String', required: true },
  lastName: { type: 'String', required: true },
  studentId: { type: 'Number', required: true },
  password: { type: 'String', required: true },
  email: { type: 'String', required: true },
  dateAdded: { type: 'Date', default: Date.now, required: true },
  lastLogin: { type: 'Date', default: null, required: false },
  studyGroups: [{ type: 'String' }],
});  

const studyGroupSchema = new Schema({
  guid: { type: 'String', required: true },
  groupName: { type: 'String', required: true },
  course: { type: 'String', required: true },
  teacher: { type: 'String', required: true },
  description: { type: 'String', required: true },
  dateAdded: { type: 'Date', default: Date.now, required: true },
  chatMessages: [{ type: 'String' }],
});

export function getUserStudyGroups(req, res) {
  User.aggregate([
    { "$unwind": "$studyGroups" },
    {
      "$lookup": {
        "from": "studyGroups",
        "localField": "studyGroups",
        "foreignField": "cuid",
        "as": "resultingStudyGroupsArray"
      }
    },
    { "$unwind": "$resultingStudyGroupsArray" },
    {
      "$group": {
        "_id": null,
        "myStudyGroups": { "$addToSet": "$resultingStudyGroupsArray" },
        "count": { "$sum": 1 }
      }
    }
  ]).exec(function(err, results){
    console.log(results);
    return res.json({ studyGroups: results });
  });
}

Answer 1

foreignfield应为GUID。

export function getUserStudyGroups(req, res) {   User.aggregate([
    { "$unwind": "$studyGroups" },
    {
      "$lookup": {
        "from": "studyGroups",
        "localField": "studyGroups",
        "foreignField": "guid",
        "as": "resultingStudyGroupsArray"
      }
    },
    { "$unwind": "$resultingStudyGroupsArray" },
    {
      "$group": {
        "_id": null,
        "myStudyGroups": { "$addToSet": "$resultingStudyGroupsArray" },
        "count": { "$sum": 1 }
      }
    }   ]).exec(function(err, results){
    console.log(results);
    return res.json({ studyGroups: results });   }); }