Join fetch not working for many-to-one associations after migration to Hibernate 6

We are on the process of migrating our apps to Spring Boot 3.0.3 and Hibernate 6.1.7 and noticed a change that affects to queries with join fetch clauses when the fetched entity is joined through a many-to-one relationship. We are using Spring Data JPA.

As a example we got the HotelRoom entity which has a many-to-one relationship with RoomType:

@Entity
@Table(name = "hotel_room")
@Getter
@Setter
public class HotelRoomEntity implements Serializable {

	@EmbeddedId
	private HotelRoomId id;
}

@Embeddable
@Data
public class HotelRoomId implements Serializable {

	@ManyToOne(fetch = FetchType.LAZY)
	@JoinColumn(name = "hotel_code", referencedColumnName = "hotel_code")
	private HotelEntity hotel;

	@ManyToOne(fetch = FetchType.LAZY)
	@JoinColumn(name = "room_type_code", referencedColumnName = "room_type_code")
	private RoomTypeEntity roomType;
}

@Entity
@Table(name = "room_type")
@Getter
@Setter
public class RoomTypeEntity implements Serializable {

	@Id
	@Column(name = "room_type")
	private String code;

	@Column(name = "instance_id")
	private Long instanceId;
}

And we have the following repository to retrieve a list of HotelRoomEntity along with their related RoomTypeEntities:

@Repository
public interface HotelRoomJpaRepository extends CrudRepository<HotelRoomEntity, HotelRoomId> {

	@Query("""
		select hotelRoom
		from HotelRoomEntity hotelRoom
		join fetch hotelRoom.id.roomType
		where hotelRoom.id.hotel.code = :hotelCode""")
	List<HotelRoomEntity> find(@Param("hotelCode") final int hotelCode);
}

Prior to the change to Hibernate 6, this generated the following query and all worked fine:

select hotelroome0_.hotel_code as hotel_code_19_0_,
hotelroome0_.room_type_code as room_type_code_19_0_,
roomtypeen1_.instance_id as instance_id2_25_1_
from HOTEL_ROOM hotelroome0_
inner join ROOM_TYPE roomtypeen1_
on hotelroome0_.room_type_code=roomtypeen1_.room_type_code
where hotelroome0_.hotel_code=?

Now, with Hibernate 6, we getting the following query and RoomTypeEntity is not fetched:

select h1_0.hotel_code,
h1_0.room_type_code
from HOTEL_ROOM h1_0
join ROOM_TYPE r1_0
on r1_0.room_type_code=h1_0.room_type_code
where hotel_code=?

For the related entity to be fetched again we must add roomType in the select clause:

select hotelRoom, roomType
from HotelRoomEntity hotelRoom
join fetch hotelRoom.id.roomType roomType
where hotelRoom.id.hotel.code = :hotelCode

Is this mandatory now with Hibernate 6? Haven't found this documented anywhere.

Seems it is an issue in Hibernate 6. Happens with embeddedId associations.

https://hibernate.atlassian.net/browse/HHH-15875

It is addressed in version 6.2 (still in development)