ruby 51 lines · 10 steps

Safe coupon redemption in Rails

A Coupon model that validates, locks, and atomically records each redemption while enforcing expiry and usage limits.

Explained by highlit

Rails Advanced validation contexts pessimistic locking scopes atomic counters concurrency

1class Coupon < ApplicationRecord
has_many :redemptions, dependent: :destroy
3 
validates :code, presence: true, uniqueness: { case_sensitive: false }
validates :usage_limit, numericality: { greater_than: 0 }, allow_nil: true
validate :not_expired, on: :redeem
validate :under_usage_limit, on: :redeem
8 
before_validation :normalize_code
10 
scope :active, -> {
  where("expires_at IS NULL OR expires_at > ?", Time.current)
    .where(disabled_at: nil)
}
15 
scope :redeemable, -> {
  active.where(<<~SQL.squish, false)
    usage_limit IS NULL OR redemption_count < usage_limit OR ?
  SQL
}
21 
def redeem!(order)
  with_lock do
    unless valid?(:redeem)
      raise ActiveRecord::RecordInvalid, self
    end
27 
    redemption = redemptions.create!(order: order)
    increment!(:redemption_count)
    redemption
  end
end
33 
private
35 
def normalize_code
  self.code = code.to_s.strip.upcase.presence
end
39 
def not_expired
  return if expires_at.blank? || expires_at.future?
42 
  errors.add(:base, "This coupon expired on #{expires_at.to_date.to_fs(:long)}")
end
45 
def under_usage_limit
  return if usage_limit.blank? || redemption_count < usage_limit
48 
  errors.add(:base, "This coupon has reached its usage limit")
end
51end

01 / 01

STEP 01

Walkthrough

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Three takeaways

1Contextual validations with `on:` let one model enforce different rules for different actions.
2Wrapping reads and writes in `with_lock` serializes concurrent redemptions so limits can't be overshot.
3Scopes can push complex eligibility logic into SQL, keeping queries efficient and reusable.

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