Rails 3: Generate Unique Codes (Coupons)

Rails 3: Generate unique codes (coupons)

You can do something like this too:

chars = ('a'..'z').to_a + ('A'..'Z').to_a
def String.random_alphanumeric(size=16)
    (0...size).collect { chars[Kernel.rand(chars.length)] }.join
end

But then you would have to compare against a database to make sure it is not used yet.

Generating unique, hard-to-guess coupon codes

The code needs to be unguessable, because the only verification you can perform before giving the user their reward is to check whether the code they entered exists in your list of "issued" codes.

• That means the number of all possible codes in that format is much larger than the number of codes you want to issue,. Depending on how easy it is to simply try codes (think of a script trying repeatedly), then you might need all possible codes to outnumber issued codes by a factor of a million or a billion or more. This sounds high, but is possible in relatively short strings.

• It also means that the codes that you use must be chosen as randomly as possible within all possible codes. This is necessary to avoid users figuring out that most valid codes start with "AAA" for example. More sophisticated users might spot that your "random" codes use a hackable random number generator (Ruby's default rand() is fast and statistically good for random data, but is hackable in this way, so don't use it).

The starting point for such a secure code would be the output from a cryptographic PRNG. Ruby has the securerandom library, which you can use to get a raw code like this:

require 'securerandom'
SecureRandom.hex
# => "78c231af76a14ef9952406add6da5d42"

This code is long enough to cover any realistic number of vouchers (millions each for everyone on the planet), without any meaningful chance of repetition or being easy to guess. However, it is a bit awkward to type from a physical copy.

Once you know how to generate a random, practically unguessable code, your next problem is understanding user experience and deciding how much you can realistically compromise security in the name of usability. You need to bear in mind the value to the end user, and therefore how hard someone might try to get a valid code. I cannot answer that for you, but can make some general points about usability:

• Avoid ambiguous characters. In print, it is sometimes difficult to see the difference between 1, I and l for example. We often understand what it is supposed to be from context, but a randomised string of characters does not have this context. It would be a bad user experience to have to try several variations of a code by testing 0 vs O, 5 vs S etc.

• Use either lower case or upper case letters but not both. Case sensitivity will not be understood or followed by some %age of your users.

• Accept variations when matching codes. Allow spaces and dashes. Perhaps even allow 0 and O to mean the same thing. This is best done by processing the input text so it is in the right case, strip separator characters etc.

• In print, separate the code into a few small parts, it will be easier for the user to find their place in the string and type a few characters at once.

• Don't make the code too long. I would suggest 12 characters, in 3 groups of 4.

• Here's an interesting one - you may want to scan the code for possible rude words, or avoid the characters that would generate them. If your code contained only the characters K, U, F, C, then there would be a high chance of offending a user. This isn't usually a concern because users do not see most computer secure codes, but these ones will be in print!

Putting that all together, this is how I might generate a usable code:

# Random, unguessable number as a base20 string
#  .rjust(12, '0') covers for unlikely, but possible small numbers
#  .reverse ensures we don't use first character (which may not take all values)
raw = SecureRandom.random_number( 2**80 ).to_s( 20 ).rjust(12, '0').reverse
# e.g. "3ecg4f2f3d2ei0236gi"

# Convert Ruby base 20 to better characters for user experience
long_code = raw.tr( '0123456789abcdefghij', '234679QWERTYUPADFGHX' )
# e.g. "6AUF7D4D6P4AH246QFH"

# Format the code for printing
short_code = long_code[0..3] + '-' + long_code[4..7] + '-' + long_code[8..11]
# e.g. "6AUF-7D4D-6P4A"

There are 20**12 valid codes in this format, which means you can issue a billion of your own codes, and there would be one in four million chance of a user simply guessing a correct one. In cryptography circles that would be very bad (this code is insecure against a fast local attack), but for a web form offering free burritos to registered users, and where you would notice a someone trying four million times with a script, it is ok.

Simple coupon generator in rails

You don't mention what sort of coupon format you require and I am sure there are a bunch of gems that do similar things. I guess one approach is to use a unique code that you can generate and then tag a user_id to the end of it to ensure uniqueness across many codes.

def generate_coupon_code(user_id)
  characters = %w(A B C D E F G H J K L M P Q R T W X Y Z 1 2 3 4 5 6 7 8 9)
  code = ''

  4.times { code << characters.sample }
  code << user_id.to_s

  code
end

How to create a coupon code generator in rails 4

Here's a small example

letters = (0..9).to_a + ('a'..'z').to_a + ('A'..'Z').to_a # add or remove possibilities
letters.sample(10).join #or any length you want

How to test uniqueness of Coupon/Promo-Codes?

Rspec's and_return method allows you to specify multiple return values that will be cycled through

For example you could write

PromoCode.stub(:generate).and_return('badcode1', 'badcode2', 'goodcode')

Which will cause the first call to generate to return 'badcode1', the second 'badcode2' etc... You can then check that the returned promocode was created with the correct code.

If you want to be race condition proof you'll want a database uniqueness constraint, so your code might actually want to be

def generate!
   create!(...)
rescue ActiveRecord::RecordNotUnique
  retry
end

In your spec you would stub the create! method to raise the first time but return a value the second time

Rails create multiple tokens for same record (multiple coupons for one offer)

It's hard to tell which parts of this you're asking for help on. Are you not sure how to generate unique coupon codes, not sure how to model it, not sure how to create multiple records at a time? I'll try to give a general answer of what I'd do if I were building this, and maybe that will be helpful to you.

I'd create two models, Offer and Coupon:

rails g model offer title:string user:references
rails g model coupon offer:references code:uuid

I don't know if you're using mysql or postgresql. I think uuid will only work on postgresql. If you're using mysql I guess make the code column a string instead. Or better yet, switch to postgresql. ;)

class Offer < ActiveRecord::Base
  belongs_to :user
  has_many :coupons
end

class Coupon < ActiveRecord::Base
  belongs_to :coupon

  before_create -> { self.code = SecureRandom.uuid }
end

I would index the database on your code column and make it unique, and disallow code from being nil. Might as well make both columns disallow nil, I'm a big believer in doing that wherever possible in the database:

class CreateCoupons < ActiveRecord::Migration
  def change
    create_table :coupons do |t|
      t.references :offer, index: true, null: false
      t.uuid :code, null: false
    end

    add_index :coupons, :code, unique: true
  end
end

Normally when you create a coupon you'd just do:

offer.coupons.create!

And it would generate the code for you in the before_create hook. But since you want to create like 100 of them at once, the simple way would be:

100.times { offer.coupons.create! }

But that's going to be kind of inefficient. I don't know of a way to get Rails to do an efficient INSERT of many records, so we're going to do it manually:

objs = []
100.times do
  objs.push "(#{offer.id}, '#{SecureRandom.uuid}', '#{Time.now}', '#{Time.now}')"
end

Coupon.connection.execute("INSERT INTO coupons (offer_id, code, created_at, updated_at) VALUES #{objs.join(", ")}")

If there's a more Railsy way of doing this, someone please let us know.

Best way to create unique token in Rails?

-- Update EOY 2022 --

It's been some time since I answered this. So much so that I've not even taken a look at this answer for ~7 years. I have also seen this code used in many organizations that rely on Rails to run their business.

TBH, these days I wouldn't consider my earlier solution, or how Rails implemented it, a great one. Its uses callbacks which can be PITA to debug and is pessimistic in nature, even though there is a very low chance of collision for SecureRandom.urlsafe_base64. This holds true for both long and short-lived tokens.

What I would suggest as a potentially better approach is to be optimistic about it. Set a unique constraint on the token in the database of choice and then just attempt to save it. If saving produces an exception, retry until it succeeds.

class ModelName < ActiveRecord::Base
  def persist_with_random_token!(attempts = 10)
    retries ||= 0
    token = SecureRandom.urlsafe_base64(nil, false)
    save!
  rescue ActiveRecord::RecordNotUnique => e
    raise if (retries += 1) > attempts

    Rails.logger.warn("random token, unlikely collision number #{retries}")
    token = SecureRandom.urlsafe_base64(16, false)
    retry
  end
end

What is the result of this?

• One query less as we are not checking for the existence of the token beforehand.
• Quite a bit faster, overall because of it.
• Not using callbacks, which makes debugging easier.
• There is a fallback mechanism if a collision happens.
• A log trace (metric) if a collision does happen
  
  • Is it time to clean old tokens maybe,
  • or have we hit the unlikely number of records when we need to go to SecureRandom.urlsafe_base64(32, false)?).

-- Update --

As of January 9th, 2015. the solution is now implemented in Rails 5 ActiveRecord's secure token implementation.

-- Rails 4 & 3 --

Just for future reference, creating safe random token and ensuring it's uniqueness for the model (when using Ruby 1.9 and ActiveRecord):

class ModelName < ActiveRecord::Base

  before_create :generate_token

  protected

  def generate_token
    self.token = loop do
      random_token = SecureRandom.urlsafe_base64(nil, false)
      break random_token unless ModelName.exists?(token: random_token)
    end
  end

end

Edit:

@kain suggested, and I agreed, to replace begin...end..while with loop do...break unless...end in this answer because previous implementation might get removed in the future.

Edit 2:

With Rails 4 and concerns, I would recommend moving this to concern.

# app/models/model_name.rb
class ModelName < ActiveRecord::Base
  include Tokenable
end

# app/models/concerns/tokenable.rb
module Tokenable
  extend ActiveSupport::Concern

  included do
    before_create :generate_token
  end

  protected

  def generate_token
    self.token = loop do
      random_token = SecureRandom.urlsafe_base64(nil, false)
      break random_token unless self.class.exists?(token: random_token)
    end
  end
end

---

Related Topics