It’s okay to make mistakes when you are up to building simple applications. But for mission critical applications like Banking/Payment Processors, you gotta be very careful and keep listening to seniors and experts in critical matters.
Lets say you are working on a feature to update the total amount deposited in a particular account. Simple code will do like
# just after amount is debited from payee's account def credit_receiver_account(debited_amount) update_attribute :total_amount, total + debited_amount end