public static Index simple(String name) {
    return new Index(name, false);
  }
@Test
  public void getJoinField_throws_ISE_on_simple_index() {
    Index underTest = Index.simple("foo");

    assertThatThrownBy(underTest::getJoinField)
      .isInstanceOf(IllegalStateException.class)
      .hasMessage("Only index accepting relations has a join field");
  }