Add support for executing derived existsBy queries.

Closes: #385
Original pull request: #386.

Author: christophstrobl

src/main/java/org/springframework/data/keyvalue/repository/query/KeyValuePartTreeQuery.java

@@ -139,6 +139,8 @@ protected Object doExecute(Object[] parameters, KeyValueQuery<?> query) {
 
 			Iterable<?> result = this.keyValueOperations.find(query, queryMethod.getEntityInformation().getJavaType());
 			return result.iterator().hasNext() ? result.iterator().next() : null;
+		} else if (new PartTree(queryMethod.getName(), queryMethod.getEntityInformation().getJavaType()).isExistsProjection()) {
+			return keyValueOperations.count(query, queryMethod.getEntityInformation().getJavaType()) > 0;
 		}
 
 		throw new UnsupportedOperationException("Query method not supported.");

src/test/java/org/springframework/data/keyvalue/repository/query/KeyValuePartTreeQueryUnitTests.java

@@ -137,10 +137,32 @@ void shouldApplyDerivedMaxResultsToQueryWithParameters() throws SecurityExceptio
 		assertThat(query.getRows()).isEqualTo(3);
 	}
 
+	@Test // GH-385
+	void shouldUseCountForExists() throws NoSuchMethodException {
+
+		when(metadataMock.getDomainType()).thenReturn((Class) Person.class);
+		when(metadataMock.getReturnType(any(Method.class)))
+				.thenReturn((TypeInformation) ClassTypeInformation.from(Boolean.class));
+		when(metadataMock.getReturnedDomainClass(any(Method.class))).thenReturn((Class) Boolean.class);
+
+		QueryMethod qm = new QueryMethod(Repo.class.getMethod("existsByFirstname", String.class), metadataMock,
+				projectionFactoryMock);
+
+		KeyValuePartTreeQuery partTreeQuery = new KeyValuePartTreeQuery(qm, QueryMethodEvaluationContextProvider.DEFAULT,
+				kvOpsMock, SpelQueryCreator.class);
+
+		KeyValueQuery<?> query = partTreeQuery.prepareQuery(new Object[] { "firstname" });
+		partTreeQuery.doExecute(new Object[] { "firstname" }, query);
+
+		verify(kvOpsMock).count(eq(query), eq(Person.class));
+	}
+
 	interface Repo {
 
 		List<Person> findByFirstname(String firstname);
 
+		boolean existsByFirstname(String firstname);
+
 		List<Person> findBy(Pageable page);
 
 		List<Person> findTop3By();