[some1else]

A simple Discount domain model with validations:

  Class Discount
    belongs_to :promo_code
    belongs_to :customer
    belongs_to :order

    validates_presence_of :promo_code, :customer, :order
    validates_associated :promo_code
    validates_uniqueness_of :promo_code_id, :scope => [:customer_id, :order_id]
  end

Limiting down to a single Promo-code per order:

  Class Discount
    # ...
    validates_uniqueness_of :order_id, :scope => :customer_id
  end

[gavingmiller]

This right here is the heart of race condition bugs, and is NOT race condition safe. When running multiple web servers and without a "validates_uniqueness_of" constraint on your database, multiple requests hitting multiple different web servers can claim multiple discounts for the same user. Problem only grows as your number of web servers grow!

[kawsper]

Read the part "Concurrency and integrity" in the Rails documentation: http://apidock.com/rails/ActiveRecord/Validations/ClassMetho...

You need to enforce the uniqueness in the DB.

[some1else]

Therefore the only thing left to do is run the following migration:

  add_index :discounts, [:promo_code_id, :customer_id, :order_id], :unique => true