[flexcoders] insert unique item

[annouss79]

hi everyone.
how can i control that a user should insert unique item in a table ?
i'm using something like this :

markingHibernateService.fill(resCheck,"flex:hql","from Marking m where
m.offre = ? and m.users = ?",param);
resCheck.addEventListener(CollectionEvent.COLLECTION_CHANGE,checkMarking);
if(taille != 0){
Alert.show("you already marked this offer , plz choose another
one","Info");
}else{
var myMarking:Marking = new Marking();
myMarking.users = usertest;
myMarking.offre = dgOfferRes.selectedItem as Offre;
myMarking.marked = "y";
myMarking.reserved = "";        markingHibernateService.createItem(myMarking);
markingHibernateService.commit();
Alert.show("succefuly marked","info");
}
public function checkMarking(event:CollectionEvent):void{
         taille = event.target.length;
}