@Test
public void testAtLeastOneFieldIsFilledExcludingIdAndOwner() throws JSONException {

    Child child = new Child("id1", "owner1", null);
    assertThat(child.isValid(), is(false));
    child.put("test1", "value1");
    assertThat(child.isValid(), is(true));
    child.remove("test1");
    assertThat(child.isValid(), is(false));
}