Rails 4 with I18n working interpolations

en.yml

en:
  errors: &errors
    messages:
      bad_uri: is an invalid url
      bad_protocol: must start with %{protocols}
  activemodel:
    errors:
      <<: *errors
  activerecord:
    errors:
      <<: *errors

uri_validator.rb

require 'addressable/uri'

# Source: http://gist.github.com/bf4/5320847
# Accepts options[:allowed_protocols]
# To make the error message work you must add :bad_uri and :bad_protocol keys to your locale file (see example locale file)
# app/validators/uri_validator.rb
class UriValidator < ActiveModel::EachValidator

  def validate_each(record, attribute, value)
    uri = parse_uri(value)
    if !uri
      record.errors.add(attribute, :bad_uri, options)
    elsif !allowed_protocols.include?(uri.scheme)
      record.errors.add(attribute, :bad_protocol, protocols: allowed_protocols_humanized)
    end
  end

private

  def allowed_protocols_humanized
    allowed_protocols.to_sentence(:two_words_connector => ' / ')
  end

  def allowed_protocols
    @allowed_protocols ||= [(options[:allowed_protocols] || ['http', 'https'])].flatten
  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 'rails_helper'

# Source: http://gist.github.com/bf4/5320847
# spec/validators/uri_validator_spec.rb
class Foo
  include ActiveModel::Validations
  attr_accessor :url
  validates :url, uri: true
end

describe UriValidator do
  subject { Foo.new }

  it 'should be valid for a valid http url' do
    subject.url = 'http://www.google.com'
    expect(subject).to be_valid
    expect(subject.errors.full_messages).to be_empty
  end

  ['http:/www.google.com', '<>hi',
   'www.google.com', 'google.com'].each do |invalid_url|
    it "#{invalid_url.inspect} is an invalid url" do
      subject.url = invalid_url
      expect(subject).not_to be_valid
      expect(subject.errors).to have_key(:url)
      expect(subject.errors[:url]).to(
        include(I18n.t(:bad_uri, scope: 'activemodel.errors.messages'))
      )
    end
  end

  ['ftp://ftp.google.com', 'ssh://google.com'].each do |invalid_url|
    it "#{invalid_url.inspect} does not respect the allowed protocols" do
      subject.url = invalid_url
      expect(subject).not_to be_valid
      expect(subject.errors).to have_key(:url)
      expect(subject.errors[:url]).to(
        include(I18n.t(:bad_protocol,
                       scope: 'activemodel.errors.messages',
                       protocols: 'http / https'))
      )
    end
  end
end