trigger trg_Invoice on Custom_Invoice__c (before insert, before update) {

// ----------------------------------------------------------------------
// 26 Jan 2009: M.Smith
// http://www.sfcnmore.com/
//
// Update a custom currency converted field using dated exchange rages
// This works around limitations with dated exchange rates on custom objects
// ----------------------------------------------------------------------

/* ......................................................................
* The following variables should be filled in for the code to work properly
* The Trigger is designed to operate generically based on the values set below
* Note: The nDecimalPlaces variable must be adjusted for the object & field as well
* ....................................................................... */
String ToCurrencyCode = 'USD';
String FIELD_CURRENCYCODE = 'CurrencyIsoCode';
String FIELD_AMOUNT = 'Invoice_Amount__c';
String FIELD_DATE = 'Invoice_Date__c';
String FIELD_CONVERTED_AMOUNT = 'Invoice_Amount_USD__c';
// --- The following two need to be set to the actual object (table) for trigger
// Have to adjust the Object and Fieldname below in order to get the decimal places.
Integer nDecimalPlaces =
    Custom_Invoice__c.Invoice_Amount_USD__c.getDescribe().getScale();
* ....................................................................... */

try {

      // A list of the Currency codes to retrieve from the DatedConversionRates table
      List<String> CurrCodes = new List<String>();
      // A list of item numbers to process from the Trigger.New[] collection
      List<Integer> TriggerItem = new List<Integer>();

      // Trigger.new is a list of the 'Objects' that were updated
      // This loop iterates over the list, and adds any that have a
      // change in the 3 key fields
      for (Integer i = 0; i < Trigger.new.size(); i++) {
        // Check to see if any of the following fields have changed:
        //  - AMOUNT field
        //  - DATE field (the date used for currency conversion)
        // If either has changed (of if this is a new record), remember
        // this record ID
        if (Trigger.IsInsert) {
           // Build a list of objects to be processed
           TriggerItem.add (i);
           CurrCodes.add( (string)Trigger.new[i].Get(FIELD_CURRENCYCODE));
        } else {
           if ((Trigger.old[i].Get(FIELD_AMOUNT) != Trigger.new[i].Get(FIELD_AMOUNT))
           || ( Trigger.old[i].Get(FIELD_DATE) != Trigger.new[i].Get(FIELD_DATE))
           || ( Trigger.old[i].Get(FIELD_CURRENCYCODE) != Trigger.new[i].Get(FIELD_CURRENCYCODE)) )
           {
              // Build a list of objects to be processed
              TriggerItem.add (i);
              CurrCodes.add( (string)Trigger.new[i].Get(FIELD_CURRENCYCODE));
           }
        }
      }

      // To allow this to work for BULK operations (> 20 records), query all of
      // the conversion rates for the currencies that are used in the records being
      // processed. This acts as a Cache to avoid querying the table for each record
      // being inserted/updated. Loop through these in memory each time to find the
      // correct rate to use
      List<DatedConversionRate> = [SELECT ISOCode, ConversionRate,
          StartDate, NextStartDate FROM DatedConversionRate
          WHERE ISOCode in :CurrCodes];

      // For each record being inserted/updated
      for ( integer i : TriggerItem)
      {
         // Retrieve the proper DatedConversionRate based on the Date fields value
         // field. Change this field name to adjust to the appropriate Custom object.
         date dDateFieldValue = (date)Trigger.new[i].Get(FIELD_DATE);
         double nRate = -1;
         for (DatedConversionRate exchRate : ExchRateCache) {
            // Looping through the cached DatedConversionRate object
            if (exchRate.ISOCode == Trigger.new[i].Get(FIELD_CURRENCYCODE)
            && exchRate.StartDate <= dDateFieldValue
            && exchRate.NextStartDate > dDateFieldValue) {
               // Look for a match for CURRENCYCODE and within the Date Ranges
               nRate = exchRate.ConversionRate;
               break;
            }
         }

         System.assertNotEquals(nRate, -1, 'A Rate was not found for the ' +
         dDateFieldValue + ' & ' + Trigger.new[i].Get(FIELD_CURRENCYCODE));
         /* System.Debug('Exchange Rate = ' + nRate); */

         // Convert the AMOUNT field to the Target Currency
         // Use .setScale() to set the decimal places accordingly
         Decimal nOriginalAmt =
            ((Decimal)Trigger.new[i].Get(FIELD_AMOUNT)).setScale(nDecimalPlaces);
         Decimal nAmount = (nOriginalAmt / nRate).setScale(nDecimalPlaces);

         // Update the field (BEFORE INSERT/UPDATE ONLY)
         Trigger.new[i].put(FIELD_CONVERTED_AMOUNT, nAmount);

      }
  } catch (Exception e) {
      // Log any errors in the debug logs
      System.Debug('Error Detected...Logic Skipped: ' + e.getMessage()
         + '/' + e.getCause());
  }

}

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