/**
* Returns expression for field unification
*
* @param string $field
* @param string $backendType
* @return \Zend_Db_Expr
*/
private function unifyField($field, $backendType = 'varchar')
{
if ($backendType == 'datetime') {
$expr = $this->connection->getDateFormatSql($field, '%Y-%m-%d %H:%i:%s');
} else {
$expr = $field;
}
return $expr;
}
