DATAMONGO-1907 - Adjust SimpleReactiveMongoRepository.findOne(…) to complete without exception on empty result

We now no longer emit an exception via SimpleReactiveMongoRepository.findOne(Example) if the query completes without yielding a result. Previously findOne(Example) emitted a NoSuchElementException if the query returned no result.

Original pull request: #541.

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/repository/support/SimpleReactiveMongoRepository.java

@@ -45,6 +45,7 @@ import org.springframework.util.Assert;
  * @author Mark Paluch
  * @author Oliver Gierke
  * @author Christoph Strobl
+ * @author Ruben J Garcia
  * @since 2.0
  */
 @RequiredArgsConstructor
@@ -97,7 +98,7 @@ public class SimpleReactiveMongoRepository<T, ID extends Serializable> implement
 						return Mono.error(new IncorrectResultSizeDataAccessException(1));
 					}
 					return Mono.just(vals.iterator().next());
-				}).single();
+				}).next();
 	}
 
 	/*

spring-data-mongodb/src/test/java/org/springframework/data/mongodb/repository/SimpleReactiveMongoRepositoryTests.java

@@ -54,6 +54,7 @@ import org.springframework.util.ClassUtils;
  *
  * @author Mark Paluch
  * @author Christoph Strobl
+ * @author Ruben J Garcia
  */
 @RunWith(SpringJUnit4ClassRunner.class)
 @ContextConfiguration("classpath:reactive-infrastructure.xml")
@@ -441,6 +442,14 @@ public class SimpleReactiveMongoRepositoryTests implements BeanClassLoaderAware,
 		StepVerifier.create(repository.findOne(example)).expectError(IncorrectResultSizeDataAccessException.class);
 	}
 
+	@Test // DATAMONGO-1907
+	public void existsByExampleShouldReturnNonExistingWithoutThrowException() {
+
+		Example<ReactivePerson> example = Example.of(new ReactivePerson("foo", "bar", -1));
+
+		StepVerifier.create(repository.findOne(example)).verifyComplete();
+	}
+
 	interface ReactivePersonRepostitory extends ReactiveMongoRepository<ReactivePerson, String> {
 
 		Flux<ReactivePerson> findByLastname(String lastname);