External Validations using only ActiveModel

draft_post_validator.rb

require 'delegate'
require 'active_model'

class DraftPostValidator < SimpleDelegator
  include ActiveModel::Validations

  validates :title, :presence => true
  validate :future_publication_date

private
  def errors
    __getobj__.errors
  end

  def future_publication_date
    errors.add(:publication_date, "must be in the future") if publication_date && publication_date <= Date.today
  end

end

gistfile1.rb

irb(main):063:0> post = Post.new
=> #<Post:0x10d196f28 @errors=#<ActiveModel::Errors:0x10d196ed8 @messages=#<OrderedHash {}>, @base=#<Post:0x10d196f28 ...>>>
irb(main):064:0> PublishedPostValidator.new(post).valid?
=> false
irb(main):065:0> post.errors.full_messages
=> ["title can't be blank", "author can't be blank"]

irb(main):070:0> post.publication_date = Date.yesterday
=> Tue, 19 Jun 2012
irb(main):071:0> DraftPostValidator.new(post).valid?
=> false
irb(main):072:0> post.errors.full_messages
=> ["title can't be blank", "publication_date must be in the future"]
irb(main):073:0> 

Post.rb

require 'active_model'

# Most of this is the basic boilerplate described in the docs for active_model/errors; ie, the bare minimum 
# a class must have to use AM::Errors 

class Post
  extend ActiveModel::Naming

  attr_reader :errors
  attr_accessor :title, :author, :publication_date

  def initialize
    @errors = ActiveModel::Errors.new(self)
  end

  def read_attribute_for_validation(attr)
    send(attr)
  end

  def self.human_attribute_name(attr, options = {})
    attr
  end

  def self.lookup_ancestors
    [self]
  end
en

published_post_validator.rb

# A Validator for published objects.  It may have more stringent validation rules than unpublished posts.

require 'delegate'
require 'active_model'

class PublishedPostValidator < SimpleDelegator
  include ActiveModel::Validations

  validates :title, :presence => true
  validates :author, :presence =>  true
  validates :publication_date, :presence => true

private
  def errors
    __getobj__.errors
  end
end

@therealadam
I was a bit perplexed by that as well, but it looks like including ActiveModel::Validations defines an errors method, so the errors get set on the validator, instead of the domain object. I didn't want that. But it's easy to miss. :|

http://api.rubyonrails.org/classes/ActiveModel/Validations.html

@cflipse
I've used the same approach, but divided into 3 parts:
domain object(data provider) - validator(defines validation rules) - validation result(stores errors)
it seemed to follow SRP a bit better