trigger InvoiceSum on Invoice_Items__c (after insert, after delete, after undelete, after update) {
    set<Id> invIdSet = new set<Id>();
    
    if(trigger.isinsert || trigger.isUpdate || trigger.Isundelete){
        for(Invoice_Items__c invItem: Trigger.new){
			if(Trigger.isInsert || Trigger.isUndelete || (invItem.Amount__c != Trigger.oldMap.get(invItem.Id).Amount__c || invItem.InvoiceFK__c != Trigger.oldMap.get(invItem.Id).InvoiceFK__c))
				invIdSet.add(invItem.InvoiceFK__c);            
        }
    }    
    if(trigger.isUpdate || trigger.isDelete) {
        for(Invoice_Items__c invItem: Trigger.old){
            if(Trigger.isDelete || (invItem.Amount__c != Trigger.newMap.get(invItem.Id).Amount__c || invItem.InvoiceFK__c != Trigger.newMap.get(invItem.Id).InvoiceFK__c))
				invIdSet.add(invItem.InvoiceFK__c);
        }
    }  
	InvoiceSumHelper.updateInvoices(invIdSet);
    
}
public class InvoiceSumHelper {
	public static void updateInvoices(set<Id> invIdSet) {
		List<Invoice__c> invList = [select id, Ammount__c, (Select Id, Amount__c from Invoice_Items__r) from Invoice__c Where ID IN: invIdSet];   
		if(invList != null && invList.size() > 0) {
			for(Invoice__c inv : invList){
				
				inv.Ammount__c = 0;
				for(Invoice_Items__c invItem : inv.Invoice_Items__r) {
					inv.Ammount__c += invItem.Amount__c;
				}
			}
			update invList;
		}
		
	}
}