Raise validation error from virtual attribute in Rails

Question

I got two models: Source and SourceType. Source of course belongs to SourceType.
I want to create new source and assign proper sourcetype object to it. 'Proper' means that one virtual attribute of the source object match some of sourceType objects test regexpression, which becomes source's Type.

I got an attribute writer in source object

class Source < ActiveRecord::Base
   belongs_to :source_type    
   def url=(value)
       SourceType.each do |type|
          # here i match type's regexp to input value and if match, 
          # assign it to the new source object
       end
   end
end

I don't want to build any custom validator for it'll be need to run through SourceTypes twice. How to raise validate error if no sourcetypes is fit to the input so user could see error reasons in a form?

Answer 1

Validation

If you set the virtual attribute using attr_accessor, you should be able to validate on the model you're sending data to (alternatively using inverse_of if you'd like to validate on the nested model):

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

This can now be used in combination with the validates method (see ActiveModel::Validations::ClassMethods.validates for more on this).

class Person
  include ActiveModel::Validations
  attr_accessor :title

  validates :title, presence: true
end

---

Code

I'd do this:

class Source < ActiveRecord::Base
   belongs_to :source_type, inverse_of: :sources
   attr_accessor :url
end

class SourceType < ActiveRecord::Base
   has_many :sources, inverse_of: :source_type
   validates :source_type_attr, presence: { if: :url_match? }

   def url_match?
      self.sources.url == [your_regex]
   end
end