I have a object that maps a table in the database. I create another object which inherits the above object with few more attributes and associations all in the same table. When i deply it it says "descriptor not specified for polymorphism". In this case idont have a descriptor as the derived object is just an extension of the base object. What do I Do?

descriptor? Most likely you mean discriminator? You allways need a discriminator for <subclass>, otherwise how would you know what object to generate for a specific row.