@Override
    protected String appendAttributeToQuery(final String queryBuilder, final String dataAttribute, final List<Object> queryValues) {
        if (queryBuilder != null) {
            return queryBuilder;
        }

        val keyAttributeName = queryAttributeName != null
            ? queryAttributeName
            : getUsernameAttributeProvider().getUsernameAttribute();

        if (keyAttributeName.equals(dataAttribute)) {
            return String.valueOf(queryValues.getFirst());
        }

        return null;
    }
The backing Map to use for queries, the outer map is keyed on the query attribute. The inner
Map is the set of user attributes to be returned for the query attribute.

@param backingMap backing map
core/cas-server-core-authentication-api/src/main/java/org/apereo/cas/authentication/attribute/ComplexPersonAttributeDao.java