@MappedSuperclass
@IdClass(PK.class)
public abstract class ParentClass implements Serializable {
    ....
}

@Entity
@Table(name="MyTable")
public class ChildClass extends ParentClass

Caused by: java.lang.IllegalArgumentException: No @IdClass attributes exist on the IdentifiableType [EntityTypeImpl@28384121:ChildClass [ javaType: class ChildClass descriptor: RelationalDescriptor(ChildClass --> [DatabaseTable(MyTable)]), mappings: 22]].  There still may be one or more @Id or an @EmbeddedId on type.