Rails with Devise: Set Unique Username from First Name

In my app I'm trying to automatically populate a username column from the first_name field that I already have tied in to my Devise login system. In theory, it should just be the user's first_name if they are the only one with that name, but it should be something like "Mal the 4th" or "Jayne the 3rd" if there are other users with that first name already.

So far, in googling and consulting other SO posts (like this one) I have this basic structure in my registrations_controller:

  before_create :set_username

  private
  def set_username
    if self.first_name ...isn't the same as that of other users...
      self.username = first_name
    else
      self.username = first_name + " the " + ordinalize(...something...)
    end
  end

But I'm struggling to fill in the blanks. So far it looks like the best way to do it is to do an if statement that checks if that first_name is unique and, if it's not to ordinalize some kind of count, but please let me know if there's a better or "more Ruby" way. Any help getting this to work would be appreciated!