invoice number generation per year

invoice.rb

class Invoice < ActiveRecord::Base
  before_validation :generate_number, on: :create

  private

  def generate_number
    prefix = "R#{Date.today.strftime('%y')}-"
    last_order_number_this_year = self.class.where("number LIKE ?", "#{prefix}%").order("number DESC").limit(1).pluck(:number).first
    number = last_order_number_this_year ? last_order_number_this_year.match(/\A#{prefix}(\d+)\z/)[1].to_i : 0

    self.number = "#{prefix}#{sprintf('%05d', number + 1)}"
  end

end