EOFetchSpecification subclass hints

My problem is to use fetch specification with hint like in this example:

         EOSQLExpression sql = 
ERXEOAccessUtilities.sqlExpressionForFetchSpecification(ec, spec, 
start, end);
         NSDictionary hints = new NSDictionary(sql, 
"EOCustomQueryExpressionHintKey");
         spec.setHints(hints);

         return ec.objectsWithFetchSpecification(spec);

This is working fine expect the fact it won't create the objects of the 
right subclass as soon as you are using an 
"EOCustomQueryExpressionHintKey" hint like in the above example.

How can I have my own sql code to fetch a table and get the right 
subclass instanciated ?


Dominique