@Override
    public Set<String> getAvailableQueryAttributes(final PersonAttributeDaoFilter filter) {
        val usernameAttributeProvider = this.getUsernameAttributeProvider();
        val usernameAttribute = usernameAttributeProvider.getUsernameAttribute();
        val list = new HashSet<String>();
        list.add(usernameAttribute);
        return list;
    }
Sets backing map.

@param backingMap the backing map
support/cas-server-support-person-directory-core/src/main/java/org/apereo/cas/persondir/MappablePersonAttributeDao.java