Beta code system.

invite_code.rb

class InviteCode < ActiveRecord::Base
  validate :invite_code, :presence => true

  def to_s
    invite_code
  end

  def self.generate
    8.times.map { [*?a..?z,*?A..?Z].sample }.join
  end
end

user.rb

class User < ActiveRecord::Base
  # ...

  attr_accessor :invite_code

  validates :invite_code, :inclusion => {
    :in => proc { InviteCode.all.map { |code| code.invite_code.to_s } },
    :message => '%{value} is not a valid invite code'
  }, :on => :create

  after_create :destroy_invite_code

  # ...

  private

  def destroy_invite_code
    InviteCode.find_by_invite_code(invite_code).destroy
  end
end