popsu-d7/sites/all/modules/references/user_reference/user_reference.install

<?php

/**
 * @file
 * Install, update and uninstall functions for the user_reference module.
 */

/**
 * Implements hook_field_schema();
 */
function user_reference_field_schema($field) {
  $columns = array(
    'uid' => array(
      'type' => 'int',
      'unsigned' => TRUE,
      'not null' => FALSE,
    ),
  );
  return array(
    'columns' => $columns,
    'indexes' => array('uid' => array('uid')),
    'foreign keys' => array(
      'uid' => array(
        'table' => 'users',
        'columns' => array('uid' => 'uid'),
      ),
    ),
  );
}