HHH-16018 Showcase a way to select one or the other ToOne assocation.

hibernate-core/src/test/java/org/hibernate/orm/test/query/hhh12225/CaseToOneAssociationTest.java

@@ -0,0 +1,97 @@
+package org.hibernate.orm.test.query.hhh12225;
+
+import org.hibernate.testing.orm.junit.EntityManagerFactoryScope;
+import org.hibernate.testing.orm.junit.JiraKey;
+import org.hibernate.testing.orm.junit.Jpa;
+import org.junit.jupiter.api.Assertions;
+import org.junit.jupiter.api.BeforeAll;
+import org.junit.jupiter.api.Test;
+
+import jakarta.persistence.Entity;
+import jakarta.persistence.FetchType;
+import jakarta.persistence.GeneratedValue;
+import jakarta.persistence.GenerationType;
+import jakarta.persistence.Id;
+import jakarta.persistence.ManyToOne;
+
+@Jpa(annotatedClasses = {
+		CaseToOneAssociationTest.Subject.class,
+		CaseToOneAssociationTest.Link.class
+})
+@JiraKey( "HHH-16018" )
+public class CaseToOneAssociationTest {
+
+	private Subject persistedFrom;
+
+	@BeforeAll
+	public void setUp(EntityManagerFactoryScope scope) {
+		persistedFrom = scope.fromTransaction( em -> {
+			final Subject from = new Subject();
+			final Subject to = new Subject();
+			em.persist( from );
+			em.persist( to );
+			em.persist( new Link(from, to) );
+			return from;
+		} );
+	}
+
+	@Test
+	public void testUnsupportedCaseForEntity(EntityManagerFactoryScope scope) {
+		// Won't catch AssertionError from assertFound, because it is Error, not Exception.
+		// No test for message or more specific type, because there is nothing really related to the attempt
+		// to select an entity in "case".
+		Assertions.assertThrows( Exception.class, () -> {
+			assertFound(
+					scope.fromTransaction( em -> em
+							.createQuery(
+									"select case when l.from = :from then l.to else l.from end from Link l where from = :from",
+									Subject.class
+							)
+							.setParameter( "from", persistedFrom )
+							.getSingleResult() )
+			);
+		});
+	}
+
+	private void assertFound(Subject found) {
+		Assertions.assertNotEquals( persistedFrom.id, found.id, "Found itself" );
+	}
+
+	@Test
+	public void testSupportedCaseForJoin(EntityManagerFactoryScope scope) {
+		assertFound(
+				scope.fromTransaction( em -> em
+						.createQuery(
+								"select s from Link l join Subject s ON s.id = case when l.from = :from then l.to.id else l.from.id end where from = :from",
+								Subject.class
+						)
+						.setParameter( "from", persistedFrom )
+						.getSingleResult() )
+		);
+	}
+
+	@Entity(name = "Subject")
+	public static class Subject {
+		@Id
+		@GeneratedValue(strategy = GenerationType.IDENTITY)
+		public Long id;
+	}
+
+	@Entity(name = "Link")
+	public static class Link {
+		@Id
+		@GeneratedValue(strategy = GenerationType.IDENTITY)
+		public Long id;
+		@ManyToOne(fetch = FetchType.LAZY)
+		public Subject from;
+		@ManyToOne(fetch = FetchType.LAZY)
+		public Subject to;
+
+		public Link() {}
+
+		public Link(Subject from, Subject to) {
+			this.from = from;
+			this.to = to;
+		}
+	}
+}