#469 - Suppress emission of null values when using simple and primitive result types.

Results projected onto simple and primitive types that are null are no longer emitted.

A SQL query SELECT MAX(age) FROM my_table that returns a SQL NULL and that would be consumed as Long.class (Publisher<Long>) is an example for a primitive type that can be null. Since Reactive Streams prohibits the propagation of null values by a Publisher to a Subscriber the only viable option is to suppress null results by wrapping the mapping function into Optional result values and filter these values later to avoid null being emitted.

src/test/java/org/springframework/data/r2dbc/core/R2dbcEntityTemplateUnitTests.java

@@ -104,7 +104,25 @@ public class R2dbcEntityTemplateUnitTests {
 		assertThat(statement.getBindings()).hasSize(1).containsEntry(0, Parameter.from("Walter"));
 	}
 
-	@Test // gh-220
+	@Test // gh-469
+	public void shouldProjectExistsResult() {
+
+		MockRowMetadata metadata = MockRowMetadata.builder()
+				.columnMetadata(MockColumnMetadata.builder().name("name").build()).build();
+		MockResult result = MockResult.builder().rowMetadata(metadata)
+				.row(MockRow.builder().identified(0, Object.class, null).build()).build();
+
+		recorder.addStubbing(s -> s.startsWith("SELECT"), result);
+
+		entityTemplate.select(Person.class) //
+				.as(Integer.class) //
+				.matching(Query.empty().columns("MAX(age)")) //
+				.all() //
+				.as(StepVerifier::create) //
+				.verifyComplete();
+	}
+
+	@Test // gh-469
 	public void shouldExistsByCriteria() {
 
 		MockRowMetadata metadata = MockRowMetadata.builder()

src/test/java/org/springframework/data/r2dbc/repository/H2R2dbcRepositoryIntegrationTests.java

@@ -92,6 +92,11 @@ public class H2R2dbcRepositoryIntegrationTests extends AbstractR2dbcRepositoryIn
 		return H2LegoSetRepository.class;
 	}
 
+	@Test // gh-469
+	public void shouldSuppressNullValues() {
+		repository.findMax("doo").as(StepVerifier::create).verifyComplete();
+	}
+
 	@Test // gh-235
 	public void shouldReturnUpdateCount() {
 
@@ -139,6 +144,9 @@ public class H2R2dbcRepositoryIntegrationTests extends AbstractR2dbcRepositoryIn
 
 	interface H2LegoSetRepository extends LegoSetRepository {
 
+		@Query("SELECT MAX(manual) FROM legoset WHERE name = :name")
+		Mono<Integer> findMax(String name);
+
 		@Override
 		@Query("SELECT name FROM legoset")
 		Flux<Named> findAsProjection();