DATAMONGO-1619 - Polishing.

Align to changes in DATACMNS-995 and emit Exception if findOne yields more than one result.

Original Pull Request: spring-projects#444

Author: christophstrobl

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

@@ -27,6 +27,7 @@
 import java.util.stream.Collectors;
 
 import org.reactivestreams.Publisher;
+import org.springframework.dao.IncorrectResultSizeDataAccessException;
 import org.springframework.data.domain.Example;
 import org.springframework.data.domain.Sort;
 import org.springframework.data.mongodb.core.ReactiveMongoOperations;
@@ -43,6 +44,7 @@
  *
  * @author Mark Paluch
  * @author Oliver Gierke
+ * @author Christoph Strobl
  * @since 2.0
  */
 @RequiredArgsConstructor
@@ -86,7 +88,16 @@ public <S extends T> Mono<S> findOne(Example<S> example) {
 		Assert.notNull(example, "Sample must not be null!");
 
 		Query q = new Query(new Criteria().alike(example));
-		return mongoOperations.findOne(q, example.getProbeType(), entityInformation.getCollectionName());
+		q.limit(2);
+
+		return mongoOperations.find(q, example.getProbeType(), entityInformation.getCollectionName()).buffer(2)
+				.flatMap(vals -> {
+
+					if (vals.size() > 1) {
+						return Mono.error(new IncorrectResultSizeDataAccessException(1));
+					}
+					return Mono.just(vals.iterator().next());
+				}).single();
 	}
 
 	/*

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

@@ -35,6 +35,7 @@
 import org.springframework.beans.factory.BeanFactory;
 import org.springframework.beans.factory.BeanFactoryAware;
 import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.dao.IncorrectResultSizeDataAccessException;
 import org.springframework.data.annotation.Id;
 import org.springframework.data.domain.Example;
 import org.springframework.data.domain.Sort;
@@ -49,9 +50,10 @@
 import org.springframework.util.ClassUtils;
 
 /**
- * Test for {@link ReactiveMongoRepository}.
+ * Tests for {@link ReactiveMongoRepository}.
  *
  * @author Mark Paluch
+ * @author Christoph Strobl
  */
 @RunWith(SpringJUnit4ClassRunner.class)
 @ContextConfiguration("classpath:reactive-infrastructure.xml")
@@ -403,6 +405,15 @@ public void existsByExampleShouldReturnNonExisting() {
 		StepVerifier.create(repository.exists(example)).expectNext(false).verifyComplete();
 	}
 
+	@Test // DATAMONGO-1619
+	public void findOneShouldEmitIncorrectResultSizeDataAccessExceptionWhenMoreThanOneElementFound() {
+
+		Example<ReactivePerson> example = Example.of(new ReactivePerson(null, "Matthews", -1),
+				matching().withIgnorePaths("age"));
+
+		StepVerifier.create(repository.findOne(example)).expectError(IncorrectResultSizeDataAccessException.class);
+	}
+
 	interface ReactivePersonRepostitory extends ReactiveMongoRepository<ReactivePerson, String> {
 
 		Flux<ReactivePerson> findByLastname(String lastname);