uri_validator.rb

require 'addressable/uri'
#Accepts options[:message] and options[:allowed_protocols]
class UriValidator < ActiveModel::EachValidator
 
  def validate_each(record, attribute, value)
    uri = parse_uri(value)
    if !uri
      record.errors[attribute] << generic_failure_message
    elsif !allowed_protocols.include?(uri.scheme)
      record.errors[attribute] << "must begin with #{allowed_protocols_humanized}"
    end
  end
 
  private
 
  def generic_failure_message
    options[:message] || "is an invalid URL"
  end
 
  def allowed_protocols_humanized
    allowed_protocols.to_sentence(:two_words_connector => 'or')
  end
 
  def allowed_protocols
    @allowed_protocols ||= Array((options[:allowed_protocols] || ['http', 'https']))
  end
 
  def parse_uri(value)
    uri = Addressable::URI.parse(value)
    uri.scheme && uri.host && uri
  rescue URI::InvalidURIError, Addressable::URI::InvalidURIError, TypeError
  end
 
end

uri_validator_spec.rb

require 'spec_helper'
# from https://gist.github.com/timocratic/5113293
 
describe UriValidator do
  subject do
    Test = Class.new do
      include ActiveModel::Validations
      attr_accessor :url
      validates :url, uri: true
    end
    Test.new
  end
 
  it "should be valid for a valid http url" do
    subject.url = 'http://www.google.com'
    subject.valid?
    subject.errors.full_messages.should == []
  end
  ['http:/www.google.com','<>hi'].each do |invalid_url|
    it "#{invalid_url.inspect} is an invalid url" do
      subject.url = invalid_url
      subject.valid?
      subject.errors.should have_key(:url)
    end
  end
end