Validate the quantity of associated objects

Question

Given this:

class User < ActiveRecord::Base
  has_many :photos
  MAX_PHOTOS = 5
  validate :quantity_of_photos

  def quantity_of_photos
    ???
  end
end

And this:

@user.photos.size  # => 5

I need this to fail:

@user.photos << Photo.create(valid_photo_attributes)

How do I do this validation?

Answer 1

Move the quantity of photos method to the Photo model:

class Photo < ActiveRecord::Base

   belongs_to :user
   validates :quantity_of_photos

    def quantity_of_photos
      if new_record? && user.photos.size >= User::MAX_PHOTOS
        errors.add_to_base "You cannot have more than #{MAX_PHOTOS} photos."
      end
    end

end

The validity of a ActiveRecord instance is determined by whether there are errors in it's errors array.