account.rb

# == Schema Information
#
# Table name: accounts
#
#  id         :bigint           not null, primary key
#  balance    :integer
#  created_at :datetime         not null
#  updated_at :datetime         not null
#

class Account < ApplicationRecord
  validate :account_balance_non_zero?

  def deposit(amount)
    self.balance += amount
    save!
  end

  def withdraw(amount)
    self.balance -= amount
    save!
  end

  private 
  def account_balance_non_zero?
    return true if balance >= 0
    errors.add(:balance, "Insufficient")
    false
  end
end