/**
  * get field type from schema
  * 
  * @param Tinebase_Backend_Sql_Abstract $backend
  */
 protected function _getFieldType($backend)
 {
     $schema = $backend->getSchema();
     if (Tinebase_Core::isLogLevel(Zend_Log::TRACE)) {
         Tinebase_Core::getLogger()->trace(__METHOD__ . '::' . __LINE__ . ' schema: ' . print_r($schema, TRUE));
     }
     $type = isset($schema[$this->_field]) || array_key_exists($this->_field, $schema) ? $schema[$this->_field]['DATA_TYPE'] : NULL;
     return $type;
 }