Hi, can someone please show me examples of how to compare old and new values using the EventRelation Object? Is this even possible? I just want to get the old values of the isParent and isInvitee.
Below is what I was trying out, but it gives me an error.

Method does not exist or incorrect signature: void get(Boolean) from the type Map<Id,SObject>

for (EventRelation er: eLst) {
        EventRelation oldOpp = Trigger.oldMap.get(er.EventId);
}