it "should validate the uniqueness of a column" do
    class User < Sequel::Model
      validations.clear
      validates do
        uniqueness_of :username
      end
    end
    User.dataset.extend(Module.new {
      def fetch_rows(sql)
        @db << sql

        if sql == "SELECT * FROM users WHERE (username = 'willy') LIMIT 1" then
          yield({:id => 1, :username => "willy", :password => "test"})
        end
      end
    })

    @user = User.new :username => "willy", :password => "anothertest"
    @user.should_not be_valid
    @user.errors.full_messages.should == ['username is already taken']

    @user.username = "pinky"
    @user.should be_valid
  end


# custom models validations

module Validation
  module ClassMethods

    def validates_uniqueness_of(*atts) # field value uniqueness validation
      opts = {
        :message => 'is already taken',
      }.merge!(atts.extract_options!)

      validates_each(*atts) do |o, a, v|
        o.errors[a] << opts[:message] unless v && !v.blank? && !o.class[a => v]
      end
    end

  end
end