From 2a70f069ad038dd096d5a0bcf275bd8fa055ceef Mon Sep 17 00:00:00 2001
From: Andrea Boriero <andrea@hibernate.org>
Date: Tue, 22 Oct 2024 16:03:28 +0200
Subject: [PATCH] HHH-17838 @OneToOne relationship + @Embeddable keys +
 FetchType.LAZY fail

---
 .../internal/ToOneAttributeMapping.java       | 40 ++++++++++++++-----
 1 file changed, 29 insertions(+), 11 deletions(-)

diff --git a/hibernate-core/src/main/java/org/hibernate/metamodel/mapping/internal/ToOneAttributeMapping.java b/hibernate-core/src/main/java/org/hibernate/metamodel/mapping/internal/ToOneAttributeMapping.java
index 2611f58fc1..3d6628cea3 100644
--- a/hibernate-core/src/main/java/org/hibernate/metamodel/mapping/internal/ToOneAttributeMapping.java
+++ b/hibernate-core/src/main/java/org/hibernate/metamodel/mapping/internal/ToOneAttributeMapping.java
@@ -1553,7 +1553,19 @@ public class ToOneAttributeMapping
 
 		having the left join we don't want to add an extra implicit join that will be translated into an SQL inner join (see HHH-15342)
 		*/
-		if ( fetchTiming == FetchTiming.IMMEDIATE && selected ) {
+
+		final ForeignKeyDescriptor.Nature resolvingKeySideOfForeignKey = creationState.getCurrentlyResolvingForeignKeyPart();
+		final ForeignKeyDescriptor.Nature side;
+		if ( resolvingKeySideOfForeignKey == ForeignKeyDescriptor.Nature.KEY && this.sideNature == ForeignKeyDescriptor.Nature.TARGET ) {
+			// If we are currently resolving the key part of a foreign key we do not want to add joins.
+			// So if the lhs of this association is the target of the FK, we have to use the KEY part to avoid a join
+			side = ForeignKeyDescriptor.Nature.KEY;
+		}
+		else {
+			side = this.sideNature;
+		}
+
+		if ( ( fetchTiming == FetchTiming.IMMEDIATE && selected ) || needsJoinFetch( side ) ) {
 			final TableGroup tableGroup = determineTableGroupForFetch(
 					fetchablePath,
 					fetchParent,
@@ -1631,16 +1643,6 @@ public class ToOneAttributeMapping
 
 		 */
 
-		final ForeignKeyDescriptor.Nature resolvingKeySideOfForeignKey = creationState.getCurrentlyResolvingForeignKeyPart();
-		final ForeignKeyDescriptor.Nature side;
-		if ( resolvingKeySideOfForeignKey == ForeignKeyDescriptor.Nature.KEY && this.sideNature == ForeignKeyDescriptor.Nature.TARGET ) {
-			// If we are currently resolving the key part of a foreign key we do not want to add joins.
-			// So if the lhs of this association is the target of the FK, we have to use the KEY part to avoid a join
-			side = ForeignKeyDescriptor.Nature.KEY;
-		}
-		else {
-			side = this.sideNature;
-		}
 		final DomainResult<?> keyResult;
 		if ( side == ForeignKeyDescriptor.Nature.KEY ) {
 			final TableGroup tableGroup = sideNature == ForeignKeyDescriptor.Nature.KEY
@@ -1692,6 +1694,22 @@ public class ToOneAttributeMapping
 		);
 	}
 
+	private boolean needsJoinFetch(ForeignKeyDescriptor.Nature side) {
+		if ( side == ForeignKeyDescriptor.Nature.TARGET ) {
+			// The target model part doesn't correspond to the identifier of the target entity mapping
+			// so we must eagerly fetch with a join (subselect would still cause problems).
+			final EntityIdentifierMapping identifier = entityMappingType.getIdentifierMapping();
+			final ValuedModelPart targetPart = foreignKeyDescriptor.getTargetPart();
+			if ( identifier != targetPart ) {
+				// If the identifier and the target part of the same class, we can preserve laziness as deferred loading will still work
+				return identifier.getExpressibleJavaType().getJavaTypeClass() != targetPart.getExpressibleJavaType()
+						.getJavaTypeClass();
+			}
+		}
+
+		return false;
+	}
+
 	private boolean isAffectedByEnabledFilters(DomainResultCreationState creationState) {
 		final LoadQueryInfluencers loadQueryInfluencers = creationState.getSqlAstCreationState()
 				.getLoadQueryInfluencers();