Unique codes

codes.rb

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

# Simple Ruby approach.
codes = []
 => [] 
codes << generate_coupon_code(123)
 => ["R5L6123"] 
codes << generate_coupon_code(123)
 => ["R5L6123", "M6EJ123"] 
new_code = generate_coupon_code(123)
 => "81PR123" 
codes << new_code if !codes.include?(new_code)
 => ["R5L6123", "M6EJ123", "81PR123"] 
codes << new_code if !codes.include?("R5L6123")
 => nil 
codes
 => ["R5L6123", "M6EJ123", "81PR123"]

# If you are using ActiveRecord consider using a callback (http://api.rubyonrails.org/classes/ActiveRecord/Callbacks.html) 
# that generates another code in the rare case that it is not unique.

class Foo < ActiveRecord::Base 
  validate :code_is_unique
 
  private

  def code_is_unique
    unless Foo.where(code: code).blank?
      errors.add(:code, 'This code is not unique. Please try again.')
    end
  end
end

# This could introduce unwated behviour for your users - as they didn't actually create his 'non unique code'.
# So perhaps you could run this...(unsure whether this is the best approach though.) 
# Disclaimer : completely untested!
Class Foo < ActiveRecord::Base
  before_validation :code_is_unique

  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

  private
  
  def code_is_unique
    unless Foo.where(code: code).blank?
      self.code = generate_coupon_code(user_id)
    end
  end
end