token_example.rb

class MyModel < ApplicationRecord
  TOKEN_CHARS = 'ybndrfg8ejkmcpqxt1wisza345h769'

  before_create :generate_token

  # Generates random token leaving out numbers and letters that might be ambiguous and that might
  # result in words some people find offensive.
  def self.typeable_token(length = 8)
    length.times.map do
      TOKEN_CHARS[SecureRandom.random_number(TOKEN_CHARS.length)]
    end.join
  end

  private
  
  def generate_token
    # A collision is extremely unlikely, but let’s try up to 4 times in case we’re very lucky.
    4.times do
      self.token = self.class.typeable_token
      break if valid?
    end
  end

  validates :token, uniqueness: true
end