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trigger updateTermOfMonthlyAmount on Opportunity (after insert, after update) {
    for (Opportunity opp : Trigger.new) {
        
        //delete old record first
        List<Term_of_Monthly_Amount__c> terms = [SELECT Id FROM Term_of_Monthly_Amount__c WHERE Opportunity__c = :opp.Id];
        if (terms.size() > 0) {
            delete terms;
        }
        
        //get month difference
        List<Term_of_Monthly_Amount__c> termToInsert = new List<Term_of_Monthly_Amount__c>();
        Decimal amountPerMonth = opp.Amount;
        Integer monthDiff = (Integer) opp.Implementation_Duration__c;
        Integer seq = 1;
        
        //loop the term of monthly amount based on frequency each product
        for (Integer i=1; i<=monthDiff; i++) {
            Term_of_Monthly_Amount__c tma = new Term_of_Monthly_Amount__c();
            tma.Sequence__c = seq;
            tma.Opportunity__c = opp.Id;
            tma.Date__c = opp.Implementation_Start_Date__c.addMonths(seq-1);
            DateTime dateDT = DateTime.newInstance(tma.Date__c.year(), tma.Date__c.month(), tma.Date__c.day(), 0,0,0);
            String monthStr = dateDT.format('MMMMM');
            tma.Period__c = monthStr;
            tma.Amount__c = amountPerMonth;
            termToInsert.add(tma);
            seq++;
        }
        
        insert termToInsert;
    }
}