generate_display_name.rb

def generate_display_name
  @display_name_candidate = generate_display_name_candidate
  if Member.find_by_display_name(@display_name_candidate)
    self.display_name = find_next_display_name_available 
  else
    self.display_name = @display_name_candidate
  end
  return true
end

  private

    def generate_display_name_candidate
      candidate = self.email.gsub(/@.*/, '').gsub(/\W/, '_')
      if candidate.length >= MIN_DISPLAY_NAME
        candidate
      else
        match_min_display_name(candidate)
      end
    end

    def match_min_display_name(candidate)
      chars_to_min_length = MIN_DISPLAY_NAME - candidate.length
      candidate + ('_' * chars_to_min_length) 
    end

    def find_next_display_name_available
      member = find_last_member_with_display_name_candidate
      if member
        appended_number = member.display_name.match(/.*-(\d+)/)[1].to_i
        @display_name_candidate + "-#{appended_number + 1}"
      else
        @display_name_candidate + '-1'
      end
    end

    def find_last_member_with_display_name_candidate
      Member.find(:last, :conditions => "display_name REGEXP '#{@display_name_candidate}-[[:digit:]]+'")
    end