A secure random token generator with tests.

example.rb

# Generating a unique auth_token for a user
#
# While a user exists with the generated token, generate a new one
Token.generate { |token| User.exists?(auth_token: token) }

token.rb

class Token

  def self.generate
    if block_given?
      loop do
        token = SecureRandom.hex
        break token unless yield(token)
      end
    else
      SecureRandom.hex
    end
  end
end

token_spec.rb

require 'rails_helper'

RSpec.describe Token do

  describe ".generate" do

    it "returns a string of random hexadecimal of length at least 32" do
      expect(Token.generate).to match(/[0-9a-f]{32,}/)
    end

    it "only returns when the given block evaluates to false" do
      n = 16

      token = Token.generate do |token|
        n = n + 1
        token.length > n
      end

      expect(token.length).to eq(n)
    end
  end
end