AddRelatedRecordOnAccount.tgr

trigger AddRelatedRecord on Account(after insert, after update) {
  List<Opportunity> oppList = new List<Opportunity>();

  // Get the related opportunities for the accounts
  Map<Id, Account> acctWithOpps = new Map<Id, Account>(
    [SELECT Id, (SELECT Id FROM opportunities) FROM Account WHERE Id
      IN :Trigger.New]);

  // Add an opportunity for each account if it doesnt already have one.
  // Iterate through each account.
  for(Account a : Trigger.New) {
    System.debug('acctsWithOpps.get(a.Id).Opportunities.size()=' +
        acctsWithOpps.get(a.Id).Opportunities.size());
    // Check if the account already has a related opportunity.
    if (acctsWithOpps.get(a.Id).Opportunities.size() == 0) {
      // If it doesnt, add a default opportunity.
      oppList.add(new Opportunity(Name=a.Name + ' Opportunity',
                                  StageName='Prospecting',
                                  CloseDate=System.today().addMonths(1),
                                  AccountId=a.Id));
    }
  }

  if (oppList.size() > 0) {
      insert oppList;
  }

}