Suppose we have the following:

Currently, Hibernate does not include InheritedKey#id3 in InheritedKeyEntity's composite key unless InheritedKey is annotated with @MappedSuperclass.

A class used as an IdClass does not need any annotations (as opposed to a class used for an EmbeddedId class, which requires @Embeddable). Since the class used as an IdClass does not need to be annotated, it seems kind of strange that its superclass would need to be annotated with @MappedSuperclass in order for its fields/properties to be persistent. Since the field/property names must match what is annotated in an IdClass, it is clear that the field/property in a superclass is intended to be included in the ID.

The fix for this issue would make annotating the superclass of a class used as an IdClass with @MappedSuperclass optional.

If the same embeddable is used as an EmbeddedId, then the embeddable’s superclasses will still require {{@MappedSuperclass}} in order for its properties to be persistent in the EmbeddedId.