Null autowired fields in test classes

[choda]

Following up on the solution provided in #144, I defined an abstract class that I use as a parent for my test classes (in a Spring Boot 1.5.3 project):

public abstract class ControllerTest {

    @Autowired
    private ApplicationContext applicationContext;
    @Autowired
    MockMvc mvc;
    @Autowired
    private Fongo fongo;

    // configure a connection to an in-memory server
    @ClassRule
    public static final InMemoryMongoDb inMemoryMongoDb = newInMemoryMongoDbRule().build();

    @Rule
    public MongoDbRule mongoDbRule = getSpringMongoDbRule();

    // Custom rule to connect to the in-memory server. For more information, see
    // https://github.com/lordofthejars/nosql-unit/issues/144
    private SpringMongoDbRule getSpringMongoDbRule() {

        MongoDbConfiguration configuration = new MongoDbConfiguration();
        configuration.setDatabaseName("test");
        // works fine if I use MockMongoClient.create(new Fongo("inMemoryInstance"));
        Mongo mongo = MockMongoClient.create(fongo);
        configuration.setMongo(mongo);

        return new SpringMongoDbRule(configuration);
    }

}

@RunWith(SpringRunner.class)
@SpringBootTest
@AutoConfigureMockMvc
public class MainControllerTest extends ControllerTest {

    @Test
    @UsingDataSet(loadStrategy = LoadStrategyEnum.DELETE_ALL)
    public void testGETRootUrl() throws Exception {
        RequestBuilder builder = MockMvcRequestBuilders.get("/")
            .accept(MediaType.APPLICATION_JSON_UTF8);

        mvc.perform(builder).andExpect(status().isOk());
    }

}

The bean I try to inject is defined in a AbstractMongoConfiguration subclass:

@Configuration
public class MongoConfiguration extends AbstractMongoConfiguration {

    @Bean
    public Fongo fongo() {
        return new Fongo("inMemoryInstance");
    }

    @Bean
    @Override
    public Mongo mongo() {
        return fongo().getMongo();
    }

    @Bean
    @Override
    protected String getDatabaseName() {
        return "test";
    }

    @Override
    protected String getMappingBasePackage() {
        return "my.base.package";
    }

    @Bean
    @Override
    public MongoTemplate mongoTemplate() {
        return new MongoTemplate(mongo(), getDatabaseName());
    }

}

When I try to inject the fongo Bean (or the database name) from the MongoConfiguration, a null value is injected and so all tests fail because of a NullPointerException.
This absolutely not critical, since if I use a new Fongo("inMemoryInstance") instead, everything works as expected and my tests run smoothly. But it's been bugging me for a few hours now and I'd really appreciate if someone could help me figure out what I'm doing wrong.