trigger to update invoice total whenever there is an adjustment change

invoice_adjustment.rb

class AddTriggerForInvoiceAmount < ActiveRecord::Migration[5.1]
  def change

    reversible do |dir|
      dir.up do
        execute <<-SQL
        CREATE OR REPLACE FUNCTION calculate_invoice_current_balance()
          RETURNS trigger AS $calc_invoice_balance$
            BEGIN
              IF (TG_OP = 'DELETE') THEN
                UPDATE invoices 
                  SET triggered_invoice_amount = (SELECT SUM(invoice_amount) 
                    FROM invoice_adjustments 
                    WHERE invoice_id = OLD.invoice_id) 
                  WHERE id = OLD.invoice_id;
                RETURN OLD;
              ELSE  /* below is for insert or update */
                UPDATE invoices 
                  SET triggered_invoice_amount = (SELECT SUM(invoice_amount) 
                    FROM invoice_adjustments 
                    WHERE invoice_id = NEW.invoice_id) 
                  WHERE id = NEW.invoice_id;
                RETURN NEW;
              END IF;
            END;
            $calc_invoice_balance$ LANGUAGE plpgsql;

        CREATE TRIGGER invoice_adjustments_added
          AFTER UPDATE OR INSERT OR DELETE
          ON invoice_adjustments
          FOR EACH ROW
          EXECUTE PROCEDURE calculate_invoice_current_balance();
        SQL
      end
      dir.down do
        execute <<-SQL
          DROP TRIGGER IF EXISTS invoice_adjustments_added ON invoice_adjustments;
          DROP FUNCTION IF EXISTS calculate_invoice_current_balance();
        SQL
      end
    end
  end
end