@Test
public void testGetAllColumnNames() {

    final String sql = "select c.name from Column c";
    final List<String> columnNameList = new ArrayList<>();
    columnNameList.add("Johanna");
    columnNameList.add("Uwe");
    final Query query = mock(Query.class);
    when(persistenceManager.createQuery(sql)).thenReturn(query);
    when(query.getResultList()).thenReturn(columnNameList);
    final List<String> namesFromStorage = columnStorage.getAllColumnNames();
    assertNotNull(namesFromStorage);
    verify(persistenceManager, times(1)).createQuery(sql);
    verify(query, times(1)).getResultList();
    verifyNoMoreInteractions(persistenceManager);
    verifyNoMoreInteractions(query);
}