Ruby on Rails - Generating random number that does not exist in database

In my application I have a Case model & every case generated by user has a case_number.

case_number is generated from my Case model. Currently Im creating a 3 digit random number from my model after creating a case.

class Case < ActiveRecord::Base

  after_create :random_case_number

  private
    def random_case_number
      random_number = Random.rand(999)
      self.update_attributes(case_number: random_number)
    end
end

This have caused so some of the cases have the same case_number. How can I from my model generate a random number that has not been currently used & if all of the 3 digit numbers are taken, it can move to 4 digits and so on.