skip_validation_in_test.rb

# app/validators/domain_validator.rb
require "resolv"
class DomainValidator < ActiveModel::EachValidator
  def validate_each(object,attribute,value)
    begin
      domain = Resolv::DNS.new.getresource(value, Resolv::DNS::Resource::IN::CNAME)
      valid = true if domain.name.to_s == "mydomain.com"
    rescue Exception => e
      valid = false
    end
    object.errors[attribute] << (options[:message] || "is not setup correctly") unless valid
  end
end

# app/models/account.rb
class Account < ActiveRecord::Base
  validates :domain,  :presence => true,
                      :uniqueness => { :case_sensitive => false },
                      :format => { :with => /^([a-z\d]+)([\-]{1}[a-z\d]+)*\.[a-z.]+$/i },
                      :domain => true
  attr_accessible :domain
end

# spec/models/account_spec.rb
# This test (and others not testing the validator above) should skip that specific validation
# for speed and to be able to use non-existing domain names

it "rejects a record if the domain is already taken" do
  Factory(:account, :domain => "yourcompany.com")
  Factory.build(:account, :domain => "yourcompany.com").should_not be_valid
end