Create your Entity

Promocode_entitty.php

/**
 * Implements hook_schema().
 */
function spaint_promocode_schema() {
  $schema['spaint_promocode'] = array(
    // example (partial) specification for table "node"
    'description' => t('The base table for nodes.'),
    'fields' => array(
      'id' => array(
        'description' => t('The primary identifier for a node.'),
        'type' => 'serial',
        'unsigned' => TRUE,
        'not null' => TRUE,
      ),
      'type' => array(
        'description' => t('The type of this entity.'),
        'type' => 'varchar',
        'length' => 32,
        'not null' => TRUE,
      ),
      'target' => array(
        'description' => t('The brushset that is activated.'),
        'type' => 'varchar',
        'length' => 32,
        'not null' => TRUE,
      ),
      'code' => array(
        'description' => t('The value of this entity.'),
        'type' => 'varchar',
        'length' => 10,
        'not null' => TRUE,
      ),
      'uid' => array(
        'description' => t('User id who used this code.'),
        'type' => 'int',
        'unsigned' => FALSE,
        'not null' => TRUE,
        'default' => 0,
      ),
      'activated' => array(
        'description' => t('Value show used or not used this code.'),
        'type' => 'int',
        'unsigned' => FALSE,
        'not null' => TRUE,
        'default' => 0,
      ),
      'expiration' => array(
        'description' => t('Value show used or not used this code.'),
        'type' => 'int',
        'unsigned' => FALSE,
        'not null' => FALSE,
        'default' => NULL,
      ),
    ),
    'primary key' => array('id'),
    'unique keys' => array(
      'code' => array('code'),
    ),
  );

  return $schema;
}


/**
 * Implements hook_entity_info().
 */
function spaint_promocode_entity_info() {
  return array(
    'spaint_promocode' => array(
      'label' => t('Promocode'),
      'entity class' => 'Entity',
      'controller class' => 'EntityAPIController',
      'base table' => 'spaint_promocode',
      'entity keys' => array(
        'id' => 'id',
        'target' => 'target',
        'code' => 'code',
      ),
      'admin ui' => array(
        'path' => 'admin/structure/spaint_promocodes'
      ),
      'access callback' => 'spaint_promocode_access',
      'module' => 'spaint_promocode',
    ),
  );
}

/**
 * Check permisson.
 */
function spaint_promocode_access($op, $entity, $account = NULL, $entity_type = 'spaint_promocode') {
  return user_access('access administration menu');
}

/**
 * Return promocode by id.
 */
function spaint_promocode_load($id) {
  $result = entity_load('spaint_promocode', array($id));
  return $result ? reset($result) : FALSE;
}