How can I write this active record? (Rails 3.1)

Question

I have three model:

• raw_coupon
• coupon
• store

If I'm looping through raw_coupons, how can I save it as a coupon only if the same coupon_code doesn't already exist for that store? Here is code to make it more clear:

raw_coupon.each do |raw_coupon|
  coupon = Coupon.new
  coupon.store_id = raw_coupon.store_id
  coupon.coupon_code = raw_coupon.coupon_code

  coupon.save if [coupon_code for this store doesn't already exist]
end

How to I write out the [coupon_code for this store doesn't already exist]

NOTE: more than one store can be using the same coupon code hence I need to check if the same coupon_code exists for that store specifically.

EDIT: Here are the model associations:

• raw_coupon: has_one :coupon
• coupon: belongs_to :raw_coupon && belongs_to:store
• store: has_many :coupons

Answer 1

Just add a unique validation to Coupon and watch out for errors when you call coupon.save :

class Coupon < ActiveRecord::Base
    validates :coupon_code, :uniqueness => { :scope => :store_id }
    #...
end

The :scope limits the uniqueness check to the coupon codes for the coupon's store_id .

You should also add a unique index to coupons on the store_id and coupon_code as an extra layer of protection.

Answer 2

you can use find_or_initialize_by helper. I wouldnt recommend find_or_create_by helper since it wont raise error if validations fail on create.

raw_coupon.each do |raw_coupon|
  coupon = Coupon.find_or_initialize_by_store_id_and_coupon_code(raw_coupon.store_id, raw_coupon.coupon_code)
  coupon.save! if coupon.new_record? #This just saves you from extra DB calls.
end

i used save! to raise an exception if your validation fails. Just FYI