Finding a thing by reference using a non ID field.

mongoose-find-by-reference-field.js

var mongoose = require('mongoose'),

    // Load User model
    User = mongoose.model('User'),

    // Create Thing schema
    thingSchema = new mongoose.Schema({

      _user: {
        type: ObjectId,
        ref: 'User',
        required: true,
        unique: true
      }

    }),
    
    Thing;

/**
 * Find a profile using the user's username
 * @param {String} username
 * @param {Function} callback
 * @return {Query}
 */
thingSchema.statics.findByUsername = function (username, callback) { 

  // Prepare the query so it can be returned
  // before asynchronous findByUsername is complete
  var query = this.findOne();
  
  // Find user by username
  User.findByUsername(username, function (error, user) {

    var scope = this;
    var args = arguments;

    if (error || !user) {
      // Maintain asynchronous behavior
      return process.nextTick(function () {
        // Maintain scope of callback
        callback.apply(scope, args);
      });
    }
    
    // Update query with user id and execute
    query.where('_user', user._id).exec(callback);
  });
  
  // Return query for optional modification at runtime
  return query;
};

Thing = mongoose.model('Thing', thingSchema);

// Usage
Thing.findByUsername('furf', function (error, thing) {});

// With optional modification
Thing.findByUsername('furf', function (error, thing) {}).populate('_user');

@kitlee just change
var query = this.findOne();
by
var query = this.find();

And results of:
User.findByUsername(username, function (error, user) { ...
by:
User.findByUsername(username, function (error, users) { if(error){ // etc } const lstUsersIds= []; users.forEach(user => { lstUsersIds.push(user ._id); }); if (lstUsersIds.length > 0) { // _id of users query.where('_id', {$in: lstUsersIds}); } }

this basically.