Hibernate. violates foreign key constraint in cach

I have following service:

@Service
@Transactional
public class blablaService{
      ....  
      @Override
      public void deleteSystemGroups(Set<Long> ids) {
          List<Terminal> terminals = terminalDao.findAll();
          Set<SystemGroup> systemGroupsToDelete = systemGroupDao.getByIds(ids);
          for (Terminal terminal: terminals) {
              terminal.getTerminalSystemGroupsSet().removeAll(systemGroupsToDelete);   // important
            terminalDao.updateTerminal(terminal);
          }
          systemGroupDao.delete(ids);  // important
      }
}

and following dao:

public boolean updateTerminal(Terminal terminal) {
  Session session = sessionFactory.getCurrentSession();
  session.update(terminal);
  session.flush();  // ATTENTION HERE!!!
  return true;
 }

Now it is working but if remove line

session.flush();

I see following error:

org.postgresql.util.PSQLException: ERROR: update or delete on table "system_group" violates foreign key constraint "fk2593ed524a9b4be6" on table "terminal_system_group"|  Detail: Key (group_id)=(2) is still referenced from table "terminal_system_group".

I understand the cause of the problem but I am not sure that my fix is good.

Can you advise better solution?