WordPressMetaQuery.php

function get_general_users(){
  // WP_User_Query arguments
  $args = array (
    'orderby'        => 'id',
    'meta_key' => 'trainer_id',
    'meta_value' => '',
    'meta_compare' => '!=',
    'order' => 'DESC'
  );    

  // The User Query
  $user_query = new WP_User_Query( $args );
  $ret = array();

  // The User Loop
  if ( ! empty( $user_query->results ) ) {
    foreach ( $user_query->results as $user ) {

      array_push($ret, $user);
      $id = $user->ID;
      $trainerID = get_user_meta($id,'trainer_id');
      $lastCertDate = get_user_meta($id,'last_certification_date');

      if (empty($trainerID)) {
        // NO ID
        echo "no trainer id";
        // add_user_meta( $id, '_level_of_awesomeness', $awesome_level);
      } else {
        // Has ID
        echo "<ul>";
        echo "<li>" . "Trainer: " . $trainerID[0] . "</li>";
        echo "<li>" . $lastCertDate[0] . "</li>";
        echo "</ul>";

      }
    }
  } else {
    // no users found
  }
  return $ret;
}