Promo Codes in Rails

When I got tasked with adding promo codes in our Rails apps, the first thing that surprised me was how few posts there were on how to do it. So here I am to fill that gap.

First! We have to create a promo codes model:

class CreatePromoCodes < ActiveRecord::Migration
  def change
    create_table :promo_codes do |t|
      t.string :code
      t.decimal :amount
      t.text :purpose

      t.timestamps
    end
  end
end

And the model:

class PromoCode < ActiveRecord::Base
  # ...
  has_and_belongs_to_many :users
  # ...
end

Then we should add some fields to a user to reference promo codes. For our case, we want to know what promo codes they have used in the past, how much credit they have, and what user referred them.

class AddFieldsToUser < ActiveRecord::Migration
  def change
    add_column :users, :credits, :decimal, default: 0.0
    add_column :users, :referring_user_id, :integer
  end
end

And the updates to the user model:

class User < ActiveRecord::Base
  # ...
  belongs_to :referring_user, :class_name => "User", :foreign_key => "referring_user_id"
  has_and_belongs_to_many :promo_codes
  # ...
end

Plus one final migration to allow the has_and_belong_to_many to work:

class CreatePromoCodeUser < ActiveRecord::Migration
  def change
    create_table :promo_codes_users, id: false do |t|
      t.belongs_to :user, index: true
      t.belongs_to :promo_code, index: true
      t.timestamps
    end
  end
end

We also have a PromoCodesController. This isn't really doing anything unique, we just need it so we can allow admins to create promo codes

class PromoCodesController < AdminController
  before_action :set_promo_code, only: [:update, :destroy]

  respond_to :html, only: [:index]
  respond_to :json, except: [:index]

  def index
    @promo_codes = PromoCode.all
    respond_with(@promo_codes)
  end

  def create
    @promo_code = PromoCode.new promo_code_params
    @promo_code.save
  end

  def update
    @promo_code.update(promo_code_params)
  end

  def destroy
    @promo_code.destroy
    render nothing: true, status: 204
  end

  private
    def set_promo_code
      @promo_code = PromoCode.find(params[:id])
    end

    def promo_code_params
      params.require(:promo_code).permit(:code, :amount, :active, :purpose)
    end
end

We then added a route that was specific to adding a promo code (within the user controller). We post to this route whenever a user adds a promo code:

def add_promo_code
  user = User.find(params[:user_id])
  # We decided the easiest way to do referring users
  # was to have codes that looked like this: MYCODE-265,
  # with 265 being the id of the referring user.
  # Not necessarily teh best solution for everyone!
    promo_code, referring_user = params[:promo_code].split('-')
    code = PromoCode.find_by_code(promo_code)
    if code
      if user.promo_codes.include?(promo_code)
        message = 'This promo code has already been used.'
      else
        user.promo_codes << code
        user.credits += code.amount
        if referring_user && user.referring_user_id.nil? && User.find_by_id(referring_user).present?
          user.referring_user_id = referring_user
        end
        if user.save
          message = 'Promo code successfully used!'
        else
          message = 'Error using promo code. Please try again or contact customer service.'
        end
      end
    else
      message = 'Not a valid promo code.'
    end
  respond_to do |format|
    format.js {  flash[:notice] = message }
  end
end

Then, whenever we are getting ready to charge a user, we add code like this:

if @user.credits > 0.0
  if @user.credits >= @price
    @meal_plan.price = 0.0
    @user.credits = @user.credits - @price
  else
    @meal_plan.price -= @user.credits
    @user.credits = 0.0
  end
end

What do referring users get? For us, they get a credit when the person they referred makes their first purchase:

def charge_user(user)
  if user.first_charge? && user.referring_user
    user.referring_user.credits += PromoCode.find_by_code(@user.promo_codes.first).amount
    user.referring_user.save
  end
end

There are a few other smaller things, but that was the bulk. If anyone has any feedback or something they would do differently, I'd love to see it in the comments!