if (mapping.getFeatureIdExpression().equals(Expression.NIL)) {
// no real feature id mapping,
// so let's find by database row id
Set<FeatureId> ids = new HashSet<FeatureId>();
FeatureId featureId = namespaceAwareFilterFactory.featureId(fId);
ids.add(featureId);
fidFilter = namespaceAwareFilterFactory.id(ids);
} else {
// in case the expression is wrapped in a function, eg. strConcat
// that's why we don't always filter by id, but do a PropertyIsEqualTo