I know there are a lot of version of this question, but I couldn't find anything helping me.

model.js

import mongoose, { Schema } from 'mongoose';

const GroupSchema = new Schema({
  name: {
    type: String,
    required: true,
    unique: true,
    minLength: [5, 'Name must be 5 characters long'],
  },
  description: {
    type: String,
    required: true,
    minLength: [10, 'Description must be 10 characters long'],
  },
  category: {
    type: String,
  },
  meetups: [{
    type: Schema.Types.ObjectId,
    ref: 'Meetup',
  }],
}, { timestamps: true });

/**
 * Create a meetup and add it to the meetups array in the group
 */
GroupSchema.statics.addMeetup = async function (id, args) {
  const Meetup = mongoose.model('Meetup');
  // We add the group id to the meetup group element
  // Finally this is the author of the meetup
  const meetup = await new Meetup({ ...args, group: id });
  // We found the group with the id provide in the url
  // And we push the meetup id in the meetups element
  const group = await this.findByIdAndUpdate(id, { $push: { meetups: meetup.id } });

  return {
    meetup: await meetup.save(),
    group,
  };
};

export default mongoose.model('Group', GroupSchema);

controller.js

export const getGroupMeetups = async (req, res) => {
  const { groupId } = req.params;

  if (!groupId) {
    return res.status(400).json({ error: true, message: 'You need to provided a group id' });
  }

  // Search for see if group exist
  const group = await Group.findById(groupId);

  if (!group) {
    return res.status(400).json({ error: true, message: 'Group not exist' });
  }

  try {
    return res.status(200).json({
      error: false,
      meetups: await Meetup.find({ group: groupId }).populate('group', 'name'),
    });
  } catch (e) {
    return res.status(400).json({ error: true, message: 'Cannot fetch meetup' });
  }
};