/**
  * Prepare database if not exists
  * @param string $class
  * @param string $rtable (relationship table, only used if class is epObjectRelation)
  * @return boolean
  */
 public function prepareDb($class, $rtable = false)
 {
     // if relation table is given, set it to runtime manager
     if ($rtable) {
         self::$em->setRelationTable($rtable);
     }
     // get class map for class
     if (!($cm = self::$em->getClassMap($class))) {
         return false;
     }
     // no table for abstract class
     if ($cm->isAbstract()) {
         return true;
     }
     return self::$db->create($cm);
 }