Add support for filtering with an optional single result

commons-business-impl/src/main/java/com/appjars/saturn/service/ConversionQueryServiceMixin.java

@@ -61,6 +61,12 @@ default List<B> filter(QuerySpec filter) {
 		return getQueryDao().filter(filter).stream().map(this::convertToBusiness).collect(Collectors.toList());
 	}
 
+	@Transactional(value = TxType.REQUIRED, rollbackOn = Exception.class)
+	@Override
+	default Optional<B> filterWithSingleResult(QuerySpec filter) {
+		return getQueryDao().filterWithSingleResult(filter).map(this::convertToBusiness);
+	}
+
 	@Transactional(value = TxType.REQUIRED, rollbackOn = Exception.class)
 	@Override
 	default long count(QuerySpec filter) {

commons-business/src/main/java/com/appjars/saturn/service/QueryService.java

@@ -65,4 +65,14 @@ public interface QueryService<T, K> {
    */
   long count(QuerySpec filter);
 
+  /**
+   * Apply filtering to entities in the system based on a certain query specification and retrieve a
+   * single result.
+   *
+   * @param filter the query specification used for filtering
+   * @return an {@code Optional} instance containing a single entity matching the query
+   *         specification if it exists; otherwise empty
+   */
+  Optional<T> filterWithSingleResult(QuerySpec filter);
+
 }

commons-data-impl/src/main/java/com/appjars/saturn/dao/jpa/ConversionJpaDaoSupport.java

@@ -110,6 +110,16 @@ default List<S> filter(QuerySpec filter) {
 		return FilterProcesor.<T, K>of(getEntityManager(), getPersistentClass()).filter(filter).stream()
 				.map(this::convertFrom).collect(Collectors.toList());
 	}
+
+	@Override
+	default Optional<S> filterWithSingleResult(QuerySpec filter) {
+		List<T> filtered = FilterProcesor.<T, K>of(getEntityManager(), getPersistentClass()).filter(filter);
+		if (filtered.size()>1) {
+			throw new IllegalStateException("Current filter returned more than one result");
+		}
+		return filtered.stream()
+				.map(this::convertFrom).findAny();
+	}
 
 	static class FilterProcesor<T extends Identifiable<K>, K extends Serializable> {
 

commons-data-impl/src/test/java/com/appjars/saturn/dao/jpa/JpaDaoSupportTest.java

@@ -110,6 +110,18 @@ void testFilter() {
 		assertEquals(10, allMinusFirst);
 	}
 
+	@Test
+	void testFilterWithSingleResult() {
+		PersonFilter pf = new PersonFilter();
+		pf.addConstraint(ConstraintBuilder.in("id", Arrays.asList(new Integer[] { persistedPerson.getId() })));
+		boolean isPresent = dao.filterWithSingleResult(pf).isPresent();
+		assertEquals(true, isPresent);
+		pf = new PersonFilter();
+		pf.addConstraint(ConstraintBuilder.in("id", Arrays.asList(new Integer[] { -1 })));
+		isPresent = dao.filterWithSingleResult(pf).isPresent();
+		assertEquals(false, isPresent);
+	}
+
 	@Test
 	void testCount() {
 		PersonFilter pf = new PersonFilter();

commons-data/src/main/java/com/appjars/saturn/dao/QueryDao.java

@@ -32,6 +32,8 @@ public interface QueryDao<T, K> {
 
 	List<T> filter(QuerySpec filter);
 
+	Optional<T> filterWithSingleResult(QuerySpec filter);
+
 	long count(QuerySpec filter);
 
 }