Trying to calculate a running total using Core Data

FROM : Felix Rodriguez
DATE : Mon Apr 21 10:06:39 2008

I am trying to figure out how to calculate a running total using core 
data. I have created an entity called Transactions that have the 
following properties.

Transactions
   amount
   balance


I am trying to calculate the balance by adding up the amount of all 
the previous balances. I have subclased NSManagedObject and 
overridden awakeFromFetch. Here is the code I have so far.

- (void)awakeFromFetch
{
   [super awakeFromFetch];
   
   double amount = [[self valueForKey:@"amount"] doubleValue];
    [self setValue:[NSNumber numberWithFloat:amount] 
forKey:@"balance"];
}

This routine only copies the amount to the balance property. I am 
thinking that I have to Query all of the previous transactions and 
calculate their totals. I know how to create a NSFetchRequest on a 
transaction object but I am unsure on how to create the NSPredicate 
object to extract all the transactions that were created prior to the 
current transaction. Thanks for any help you can provide.