bulk trigger field update salesforce - triggers

Question

I'm trying to write a trigger where inserting, updating, or deleting a record(line item) will update an amount field. Now all this records will have same ParentID (expense) and Name(of the line items). Basically duplicate records except the Contact name will be different. So, when I add a new line item with new amount with same Parents and name, the trigger should go off and query all the line items with same Parent and should re-calculate the amount.
So, if I enter first line item and say Total amount should be 100. Then I enter 2nd line Item, trigger should fire and update Amount on both record saying '50.00'. For some reason, my trigger is not updating even though it's calculating it correctly. Where is the bug? Please help!!!
trigger Test on Expense_Line_Item__c (after insert, after update, after delete) {
set<id>testlist = new set<id>();
//List<Expense_Line_Item__c> listItem= new List<Expense_Line_Item__c>();
for (Expense_Line_Item__c a : trigger.new) {
testlist.add(a.expense__c);
}
list<Expense_Line_Item__c> mapParent =
new list<Expense_Line_Item__c>([SELECT name,
id,
Amount__c
FROM Expense_Line_Item__c
WHERE expense__c IN:testlist]);
Decimal Total = 0.0;
Integer Count = 0;
for (Expense_Line_Item__c exp : mapParent) {
Total = Total + exp.Amount__c;
Count++;
System.debug('Total during iterator::::::::::::::::::::::' + Total);
System.debug('Counter:::::::::::::::::::::::::::::::::::::' + Count);
}
if (Count > = 1)
Total = Total / Count;
System.debug('Total count after division::::::::::::::::::::::' + Total);
List <Expense_Line_Item__c> insertLineItem = new List <Expense_Line_Item__c>();
for (Expense_Line_Item__c lineItem : MapParent) {
lineItem.Amount__c = Total;
//insertLineItem.add(lineItem);
//System.debug('LineItem Amount getting inserted::::::::::::::::::::::'+lineItem.Amount__c);
}
// upsert insertLineItem;
}

Answer 1

It looks like your code is an After trigger. As you're in after trigger context you have to do DML to update the records.
It also seems to me you're trying to do too much in a single trigger, its always better to take each method individually so that you can fully control what you're doing.
I see you do have an Upsert DML statement commented out but you'd be better off using Update for this particular instance as you're not using an external id or making a decision to create or insert which is the purpose of upsert