How to avoid duplicate entries on self generated hash (edit_code in this example)

idea.rb

# model

  before_create :set_edit_code

  def self.friendly_token
    ActiveSupport::SecureRandom.hex(10)
  end
  
  def self.edit_code_exists?(edit_code)
    !where(:edit_code => edit_code).first.nil?
  end

  def set_edit_code
    loop do
      self.edit_code = self.class.friendly_token
      break self.edit_code unless self.class.edit_code_exists?(self.edit_code)
    end
  end

# rspec

  describe "#edit_code" do
    before(:each) do
      @idea = Idea.new(valid_attributes)
    end
    context "before create" do
      it "should set edit code" do
        expect {
          @idea.save
        }.should change(@idea, :edit_code).from(nil)
        @idea.edit_code.length.should == 20
      end
      it "should avoid same token" do
        @idea.save
        @idea.update_attribute(:edit_code, 'first-hash')
        Idea.should_receive(:friendly_token).and_return('first-hash', 'second-hash')
        @new_idea = Idea.new(valid_attributes)
        @new_idea.save
        @new_idea.edit_code.should == 'second-hash'
      end
    end
  end