// SpecialUserGroup(!)
SpecialUserGroup userGroup = newInstance(SpecialUserGroup.class);
userGroup.setName("special user group");
// User in SpecialUserGroup
User user = newInstance(User.class);
user.setUserGroup(userGroup);
// do mapping to UserPrime
UserPrime userPrime = mapper.map(user, UserPrime.class);
// check class type of mapped group, should be SpecialUserGroupPrime!