title: Testing reactive repository java.lang.AssertionError: expectation "assertNext" failed (expected: onNext(); actual: onComplete())
body: <p>I'm writing a simple reactive app, using spring5 and mongo reactive repositories. I wanted to test repos, followed tutorials, but still have problem:</p>
  
  <pre><code>java.lang.AssertionError: expectation "assertNext" failed (expected: onNext(); actual: onComplete())
  </code></pre>
  
  <p>Here is my entity:</p>
  
  <pre><code>@Document(collection = "products")
  @TypeAlias("product")
  @Getter
  @Builder
  @AllArgsConstructor
  @EqualsAndHashCode
  public class Product {
  
      @Id
      private ObjectId _id;
      private String productName;
      private Integer quantityPerUnit;
      private BigDecimal unitPrice;
      private Integer unitsInStock;
      private Boolean discount;
      private String categoryName;
  
      private Supplier supplier;
  
  }
  </code></pre>
  
  <p>repo, nothing special:</p>
  
  <pre><code>public interface ProductRepository extends ReactiveMongoRepository&lt;Product, String&gt; {
  }
  </code></pre>
  
  <p>and finally test: </p>
  
  <pre><code>@DataMongoTest
  @RunWith(SpringRunner.class)
  public class ProductRepositoryTest {
  
      @Autowired
      private ProductRepository productRepository;
      @Test
      public void shouldReturnOneProductWithExpected() {
          String expectedId = "54759ab3c090d83494e2d804";
          productRepository.save(first).block();
          Mono&lt;Product&gt; product = productRepository.findById(Mono.just("54759ab3c090d83494e2d804"));
  
          StepVerifier
                  .create(product)
                  .assertNext(prod -&gt; {
                      assertNotNull(prod);
                      assertThat(prod.get_id(), is(equalTo(expectedId)));
                  })
                  .expectComplete()
                  .verify();
      }
  
  
  }
  </code></pre>
  
  <p>Does anyone have idea why it's not working?</p>