Mongoose find duplicates by field

findDuplicates.js

const { User } = require("src/db");

/**
 * Find all duplicates emails in the User
 * mongoose model (mapping users collection)
 *
 *
 * This aggregation function will groups the
 * documents comparing emaiils, sanitized
 * to lowercase; it adds the "_id" fields to
 * a set; and creates a count for the # of
 * emails.
 *
 * After the first group, it matches the results
 * filtering only the results with count > 1
 *
 * At the end of the function, it returns an
 * Array of "_id"s
 */
const { User } = require("src/db");

const findDuplicatedEmailsIds = () => {
  new Promise((resolve, reject) =>
    User.aggregate([
      {
        $group: {
          _id: { email: { $toLower: "$email" } },
          ids: { $addToSet: "$_id" },
          count: { $sum: 1 }
        }
      },
      {
        $match: {
          count: { $gt: 1 }
        }
      }
    ])
  ).exec((err, res) => {
    if (err) reject(err);

    const ids = [];
    res.forEach(r => {
      r.ids.shift();
      r.ids.forEach(id => ids.push(id));
    });
  });
};

module.exports = findDuplicates;