Although Force.com has many different limits on things such as number of fields, number of specific types of fields, and those dictating how custom code should be written and can be used, there are ways to work around many of these limitations. Let's take a look at how to write a roll-up summary formula in a trigger, which we will take advantage of if we have reached the maximum allowed on our object, or want to cross several relationship bridges, or want to mimic the roll-up functionality through a lookup relationship rather than Master-Detail. In order to demonstrate this functionality, we will do two Apex roll-ups using aggregate queries to Rental Agreement; one will be based on Statement__c (Total Invoiced) and the other will be based on Payment__c (Total Payments). Create the class before the triggers and this time around rollup_Methods, as shown:

public class rollup_Methods {

  //for rolling up statements to total invoiced
  public static void rollupStatementsToRentalAgreement...