Isolating Validations in ActiveModel

I’m working on a rails project where I’m experimenting with a much more object-oriented approach than Rails usually encourages. I’m intending on writing more in depth about that, when I can better gather my thoughts about the experience. I found something recently that I can write a brief post about and hopefully get the ball rolling.

I’m trying to keep a fairly hard-line divide between my persistence layer and the actual domain objects. What this means for today’s purposes is that my domain objects know nothing about ActiveRecord (and some of them never will).

This lead me to a philisophical quandry when trying to figure out validations. My domain objects are already imbuded with ActiveModel attributes, because they communicate with the view layer in ActionPack. (ActiveModel is, essentially, the reference API that ActionPack expects.) However, validations can cross both persistence and domain concerns. I’ve also been pretty heavily burned in the past by the mess that is conditional validations.

In debating whether to embed my validations in the domain model, or in the ActiveRecord objects that my Repositories map the domain entites to, I realized that I had a third option: Separate the validations. Structuring the validations as their own class opens up a lot of the traditional OO flexibility that I’ve been aiming for in this application, and allows me to ignore the validations in situtations where they are not needed, without a lot of stub/mock gymnastics.

After a little bit of experimentation, I found that we’re even able to structure the class so it uses the validation DSL we’re all used to from within ActiveRecord. There are a couple of rough edges in the code below, but it gives a good run at a proof of concept.

First, the domain model. Most of this is a straightforward copy of “minimal” required by the documentation in ActiveModel::Errors

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
end

Next, we can build a validation. In this case, I want to be able to validate a post in two different states – draft and published. A published post requires an author, a title, and a publication date:

# 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

while a draft post only requires a title, but also requires that the publication date, if set, be sometime in the future:

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

These work pretty well, using a trick with SimpleDelegator where the validation objects wrap and forward unknown methods, such as data accessors, to the domain model. Since mixing in ActiveModel::Validations provides a #valid? method to the validator object, that is not forwarded.

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> 

Unfortunately, mixing in ActiveModel::Validations also adds a #errors method to the validator, meaning that any errors found get added to the validator object, instead of the domain object. If, once wrapped and validated, we only ever pass the wrapped domain object, then this is possibly fine. I don’t particularly want to have to pass the whole validator package around, so I want the errors to go down to the domain object. This is the reason for the __getobj__.errors method in the validators above.

Also worth noting is that the validators here only go one level deep; if we’re looking to wrap and decorate, we should probably throw a few supers onto a couple of methods so that the validations will actually cascade. I’m not currently interested in nesting the validations, so I’ll leave that as an exercise for anyone interested.