Как решить проблему "идентификатор столбца не уникален"?

Я создаю url ссылками из устаревшей руководство, которое означало много отладки. В первый раз я побежал сокращенным, он работал.

Теперь, при отправке ссылки, он возвращает эту ошибку: 'идентификатор столбца не является уникальным'

 private 
      def self.create_link(original)
        url = Url.create(:original => original)
        if Link.first(:identifier => url.id.to_i.to_s(36)).nil? or !DIRTY_WORDS.include? url.id.to_i.to_s(36)
          link = Link.new(:identifier => url.id.to_i.to_s(36))
          link.url = url
          link.save
          return link
        else 
          create_link(original)
        end
      end
    end 

Ошибка относится именно к этой строке.

link.save

Заранее спасибо за любую помощь!

Таковы модели.

class Url
  include DataMapper::Resource 
  property  :id,        Serial
  property  :original,  String, :length => 255
  belongs_to    :link
end



class Link 
  include DataMapper::Resource 
  property :identifier, String, :key => true
  property :created_at, DateTime
  has 1, :url 
  has n, :visits

  def self.shorten(original, custom=nil)
    url = Url.first(:original => original)
    return url.link if url
    link = nil
    if custom
      raise 'Someone has already taken this custom URL, sorry' unless 
  Link.first(:identifier => custom).nil?
      raise 'This custom URL is not allowed because of profanity' if 
  DIRTY_WORDS.include? custom
      transaction do |txn|
        link = Link.new(:identifier => custom)
        link.url = Url.create(:original => original)
        link.save
      end
    else 
      transaction do |txn|
        link = create_link(original)
      end
    end 
    return link 
  end 

  private 
  def self.create_link(original)
    url = Url.create(:original => original)
    if Link.first(:identifier => url.id.to_i.to_s(36)).nil? or !DIRTY_WORDS.include? url.id.to_i.to_s(36)
      link = Link.new(:identifier => url.id.to_i.to_s(36))
      link.url = url
      link.save
      return link
    else 
      create_link(original)
    end
  end
end 


class Visit
  include DataMapper::Resource

  property  :id,            Serial
  property  :created_at,    DateTime
  property  :ip,            IPAddress
  property  :country,       String
  belongs_to    :link

  after :create, :set_country

  def set_country
    xml = RestClient.get "http://api.hostip.info/get_xml.php?ip=#{ip}"
    self.country = XmlSimple.xml_in(xml.to_s, { 'ForceArray' => false})['featureMember']['Hostip']['countryAbbrev']
    self.save
  end

  def self.count_days_bar(identifier,num_of_days)
    visits = count_by_date_with(identifier, num_of_days)
    data, labels = [], []
    visits.each {|visit| data << visit[1]; labels << "#{visit[0].day}/#{visit[0].month}" }
    "http://chart.apis.google.com/chart?chs=820x180&cht=bvs&chxt=x&chco=a4b3f4&chm=N,000000,0,-1,11&chxl=0:|#{labels.join('|')}&chds=0,#{data.sort.last+10}&chd=t:#{data.join(',')}"
  end

  def self.count_country_chart(identifier,map)
    countries, count = [], []
    count_by_country_with(identifier).each {|visit| countries << visit.country; count << visit.count}
    chart = {}
    chart[:map] = "http://chart.apis.google.com/chart?chs=440x220&cht=t&chtm=#{map}&chco=FFFFFF,a4b3f4,0000FF&chld=#{countries.join('')}&chd=t:#{count.join(',')}"
    chart[:bar] = "http://chart.apis.google.com/chart?chs=320x240&cht=bhs&chco=a4b3f4&chm=N,000000,0,-1,11&chbh=a&chd=t:#{count.join(',')}&chxt=x,y&chxl=1:|#{countries.reverse.join('|')}"
    return chart
  end 

  def self.count_by_date_with(identifier,num_of_days)
    visits = repository(:default).adapter.query("SELECT date(created_at) as date, count(*) as count FROM visits where link_ = '#{identifier}' and created_at between CURRENT_DATE-#{num_of_days} and CURRENT_DATE+1 group by date(created_at)")
    dates = (Date.today-num_of_days..Date.today)
    results = {}
    dates.each { |date|
      visits.each { |visit| results[date] = visit.count if visit.date == date }
      results[date] = 0 unless results[date]
    }
    results.sort.reverse 
  end 

  def self.count_by_country_with(identifier)
    respository(:default).adapter.query("SELECT country, count(*) as count FROM visits where link_identifier = '#{identifier}' group by country")
  end
end