We have seen multiple cases of users migrating from Hibernate 5 to 6 getting errors caused by having an invalid mapping: entity collections annotated with a mappedBy which references a property of a different type of the original entity. For example:

    @Entity
	public static class EntityA {
		@Id
		private Long id;

		@OneToMany( mappedBy = "three" )
		private List<EntityB> twos;
	}

	@Entity
	public static class EntityB {
		@Id
		private Long id;

		@ManyToOne
		private EntityC three;
	}

	@Entity
	public static class EntityC {
		@Id
		private Long id;
	}

This type of mappings used to work in Hibernate 5 (with unexpected results), but in 6 it causes errors.

We should check the mappedBy property type and give a give a more informative error message.