Mongoose aggregate returning unwanted empty array

I have two models; one for the User and the other one for the StudyGroup. Each StudyGroup has a unique guid field. The User model has a field of studyGroups that is an array of strings guid. A user can join multiple study groups.

User model

 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' }],
});  

StudyGroup model

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' }],
});

I want to find all studyGroups in the studyGroup array of the User model using the strings guid stored inside the array. Then send the corresponding studyGroup objects to the Front-end.

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

However, the code above just returns me an empty array. But I want to return an array of studyGroup objects.

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