class EventA {
   static belongsTo = [offer: Offer]
}
class EventB extends Event {}
class EventC extends Event {}

class Offer {
 static hasMany [events: Event]
}

SELECT *
  FROM OFFER O
  LEFT JOIN EVENTC C ON O.event_id = C.id
 WHERE C.ID IS NULL

def offers = Offer.list()
    for (Offer o in offers) {
       for(Event e : o.events) {
          if (e.instanceOf(EventC)) {
              // no bueno
          }
       }
    }