Testing a REST endpoint with Spring, MongoDB using ObjectIds

Question

I'm new to MongoDB and I'm writing a series of unit tests for a Mongo-backed REST web-service. Here's a simple test for a /clients/{id} enpoint :

@RunWith(MockitoJUnitRunner.class)
public class ClientsControllerMockMvcStandaloneTest {

    private MockMvc mvc;

    @Mock
    private ClientsRepository clientsRepository;

    @Mock
    private ModelMapper modelMapper;

    @InjectMocks
    private ClientsController clientsController;

    private ExceptionHandlerExceptionResolver createExceptionResolver() {

        ExceptionHandlerExceptionResolver exceptionResolver = new ExceptionHandlerExceptionResolver() {

            @SuppressWarnings("ConstantConditions")
            @Override
            protected ServletInvocableHandlerMethod getExceptionHandlerMethod(final HandlerMethod handlerMethod,
                                                                              final Exception exception) {

                final Method method = new ExceptionHandlerMethodResolver(RestResponseEntityExceptionHandler.class)
                        .resolveMethod(exception);

                final RestResponseEntityExceptionHandler handler = new RestResponseEntityExceptionHandler();

                return new ServletInvocableHandlerMethod(handler, method);
            }
        };

        exceptionResolver.getMessageConverters().add(new MappingJackson2HttpMessageConverter());
        exceptionResolver.afterPropertiesSet();

        return exceptionResolver;
    }

    @Before
    public void setup() {

        JacksonTester.initFields(this, new ObjectMapper());

        mvc = MockMvcBuilders.standaloneSetup(clientsController)
                .setHandlerExceptionResolvers(createExceptionResolver())
                .build();
    }

    // GET /api/clients/{id} 200

    @Test
    public void findById_ClientEntryFound_ShouldReturnFoundClientEntry() throws Exception {

        final ObjectId id = new ObjectId();

        final Client client = Client.builder()
                .id(id)
                .name("Microsoft")
                .build();

        final ClientDTO clientDTO = ClientDTO.builder()
                .id(id)
                .name("Microsoft")
                .build();

        when(clientsRepository.findById(id))
                .thenReturn(Optional.of(client));

        when(modelMapper.map(client, ClientDTO.class))
                .thenReturn(clientDTO);

        mvc.perform(get("/clients/" + id.toString())
                .accept(TestUtils.APPLICATION_JSON_UTF8))
                .andExpect(content().contentType(TestUtils.APPLICATION_JSON_UTF8))
                .andExpect(status().isOk())
                .andExpect(jsonPath("$.id", is(id)))
                .andExpect(jsonPath("$.name", is("Microsoft")))
                .andDo(MockMvcResultHandlers.print());

        verify(modelMapper, times(1)).map(client, ClientDTO.class);
        verify(clientsRepository, times(1)).findById(id);

        verifyNoMoreInteractions(clientsRepository);
    }
}

I expect this to work but I'm getting the following :

java.lang.AssertionError: JSON path "$.id"
Expected: is <5c9b9a0289d2b311b150b92c>
     but: was <{timestamp=1553701378, machineIdentifier=9032371, processIdentifier=4529, counter=5290284, timeSecond=1553701378, time=1553701378000, date=1553701378000}>
Expected :is <5c9b9a0289d2b311b150b92c>
Actual   :<{timestamp=1553701378, machineIdentifier=9032371, processIdentifier=4529, counter=5290284, timeSecond=1553701378, time=1553701378000, date=1553701378000}>
 <Click to see difference>

Any help would be appreciated (including any pointers if you think my general approach could be improved!).

Cheers!

Answer 1

Jackson doesn't know your ObjectId instance should be serialized as 5c9b9a0289d2b311b150b92c and not as:

{
  "timestamp": 1553701378,
  "machineIdentifier": 9032371,
  "processIdentifier": 4529,
  "counter": 5290284,
  "time": 1553701378000,
  "date": 1553701378000,
  "timeSecond": 1553701378
}

Luckily it's easy to fix. The ObjectId#toString() method (which will internally invoke ObjectId#toHexString()) allows you to convert the ObjectId instance into a 24-byte hexadecimal string representation.

So you could use @JsonSerialize along with ToStringSerializer to have the ObjectId instance represented as a string:

@JsonSerialize(using = ToStringSerializer.class)
private ObjectId id;

Then, in your test, use the ObjectId#toString() method (or ObjectId#toHexString()) for the assertion:

.andExpect(jsonPath("$.id", is(id.toString())))

---

Alternatively, assuming that you are using Spring Data for MongoDB, instead of ObjectId, you could use:

@Id
private String id;

You also could handle the conversion of ObjectId to String in your mapper layer.