User cannot use coupon more than once with MemberPress

memberpress-use-coupon-once-per-user.php

<?php
function custom_validate_coupon($errors) {
  global $wpdb;

  //If user is not logged in, then they've never used a coupon before
  if(!MeprUtils::is_user_logged_in()) { return $errors; }

  $coupon_code = (isset($_POST['mepr_coupon_code']) && !empty($_POST['mepr_coupon_code']))?stripslashes($_POST['mepr_coupon_code']):'';

  if(($coupon = MeprCoupon::get_one_from_code($coupon_code)) && isset($coupon->ID) && $coupon->ID) {
    $user = MeprUtils::get_currentuserinfo();

    $q = "SELECT COUNT(*) FROM {$wpdb->prefix}mepr_transactions WHERE coupon_id = {$coupon->ID} AND user_id = {$user->ID} AND status in ('complete','confirmed')";

    $count = $wpdb->get_var($q);

    if($count) {
      $errors[] = "This coupon code has already been used";
    }
  }

  return $errors;
}
add_filter('mepr-validate-signup', 'custom_validate_coupon');