@Test
  public void addToList() throws Exception {
    String id = "i1";
    ImmutableItem item = ImmutableItem.builder().id(id).addList("l1").build();

    check(repository.upsert(item).getUnchecked()).is(1);

    push(id, Collections.singleton("l2"));
    check(findById(id).list()).hasAll("l1", "l2");

    push(id, Arrays.asList("l3", "l4"));
    check(findById(id).list()).hasAll("l1", "l2", "l3", "l4");
  }
Checks {@code $push} functionality of repository
mongo/test/org/immutables/mongo/fixture/SimpleUpdaterTest.java