protected function _insert(Model_DomainObjectAbstract $obj)
{
$db = $this->getDb();
$data = array('username' => $obj->getUsername(), 'password' => $obj->getPassword(), 'firstname' => $obj->getFirstname(), 'lastname' => $obj->getLastname(), 'email' => $obj->getEmail());
$db->insert('users', $data);
}