@Test(expected = IllegalArgumentException.class)
 public void shouldThrowExceptionOnIncorrectNameForOperatorType() throws Exception {
   OperatorType.fromString("foo");
 }
 @Test
 public void shouldReturnOperatorTypeForEqualOperator() throws Exception {
   assertThat(OperatorType.EQUAL).isEqualTo(OperatorType.fromString("$eq"));
 }