Remove duplication in permission handling: * Use execute() as a Template Method, with permission() and handle() as the hook methods. * Provide a default implementation for the permission() method, which checks for self.permissionType, if the attribute is defined. * New hasPermission() method which checks whether the current user has a permission on the current class editItemPermission method: * Better error message when user is editing roles when they shouldn't be. * Extracted isEditingSelf(), which checks whether a user is editing his/her own details. * Use hasPermission method.