Duplicated Validation Across Form Objects and Models - ruby-on-rails

Question

Where do the basic validators lie when dealing with Form objects and regular Rails models?
Following the concept of decoupling forms from the persistence layer in Rails. I've setup a Form Object Cage that creates two objects together... say Animal and Plant.
Following Form Object examples from http://blog.codeclimate.com/blog/2012/10/17/7-ways-to-decompose-fat-activerecord-models/ or https://github.com/solnic/virtus or https://github.com/makandra/active_type , each of these show the Form object itself has validations... no problem... part of the benefits include being able to validate objects in a more contextually aware way.
The issue:
class Animal < ActiveRecord::Base
validates :color, presence: true
validate :only_one_brown
private
def only_one_brown
if some_complex_thing
errors.add(:color, 'can not have more than one brown animal.')
end
end
end
class Plant < ActiveRecord::Base
validates :color, presence: true
end
class Cage
include Virtus.model # or ActiveType or whatever
include ActiveModel::Validations
attribute :bird_color, String
attribute :plant_color, String
validates :bird_color, presence: true
validates :plant_color, presence: true
def save
if valid?
animal.save!
plant.save!
true
else
false
end
end
def animal
#animal ||= Animal.new(color: bird_color)
end
def plant
#plant ||= Plant.new(color: plant_color)
end
end
How do I validate animal's "only one brown" rule without:
Too much duplication.
A lot of code to make Cage still act like an AR model
If we don't duplicate the validation code, when "only one brown" is false, Cage doesn't have an error for it... we'll raise, which requires the controller to catch and handle, which is bad.
If we do duplicate the code, and if there are several custom validations, we're duplicating a lot of code and each other form object that deals with Animal needs the duplicated validations now.
If we move the validation code out of Animal into Cage entirely, similar issue: all objects that interact with Animal need to know about the "only one brown" rule, which is just duplicating validators and opening up an easy way to forget to enforce it somewhere.
If we move Animal's error array up to Cage's, Animal's error is on :color, which is ambiguous to Cage, and shows an error on an attribute name the client never sent in. If you want to map Animal's error keys to Cage's, now you need to keep an map for each Form Object, feels stinky.
Are there any good patterns or ways to deal with this situation? I feel like it is very common when you start using Form Objects but all examples are quite trivial.
Thanks in advance!

Answer 1

At the end of point 3 on http://blog.codeclimate.com/blog/2012/10/17/7-ways-to-decompose-fat-activerecord-models/ the author says: "As a bonus, since validation logic is often contextual, it can be defined in the place exactly where it matters instead of needing to guard validations in the ActiveRecord itself." I'm agree with Bryan Helmkamp, puts the validation where it matters, you don't need to duplicate it.
edited:
If I were you, I'll put the validation only on the ActiveRecord model. And I'll update the Cage class:
def save
if valid?
ActiveRecord::Base.transaction do
animal.save!
plant.save!
end
true
else
false
end
rescue Exception => exception
raise if valid?
false
end
And I'll add an errors method that returns the errors of Cage, Plant and Animal instances.
edited:
I think you can redefine the valid? method, and then errors works fine:
class Cage
include ActiveModel::Model
def valid_with_mymodels?
valid_without_mymodels? && animal.valid? && plant.valid?
animal.errors.each do |attribute, error|
self.errors.add :"bird_#{attribute.to_s}", error
end
plant.errors.each do |attribute, error|
self.errors.add :"plant_#{attribute.to_s}", error
end
errors.empty?
end
alias_method_chain :valid?, :mymodels
...
end
Just, be careful with the name of your attrs.
I'm not sure how works Virtus, with Rails 4 you can use ActiveModel::Model, if using rails 3 I need research.
edited:
If you are using Rails 3.2, you can't use ActiveModel::Model, but you get the same with this:
class Cage
extend ActiveModel::Naming
include ActiveModel::Conversion
include ActiveModel::Validations
...
end