Trouble with accepts_nested_attributes_for on validating foreign key - ruby-on-rails

Question

I am using Ruby on Rails v3.2.2. I would like to solve the issue related to the validation of a foreign key when using accepts_nested_attributes_for and validates_associated RoR methods. That is, I have following model classes:
class Article < ActiveRecord::Base
has_many :category_associations, :foreign_key => 'category_id'
accepts_nested_attributes_for :category_associations, :reject_if => lambda { |attributes| attributes[:category_id].blank? }
validates_associated :category_associations
end
class CategoryAssociation < ActiveRecord::Base
belongs_to :article, :foreign_key => 'article_id'
belongs_to :category, :foreign_key => 'category_id'
validates :article_id, :presence => true
validates :category_id, :presence => true
end
... and I have following controller actions:
class ArticlesController < ApplicationController
def new
#article = Article.new
5.times { #article.category_associations.build }
# ...
end
def create
#article = Article.new(params[:article])
if #article.save
# ...
else
# ...
end
end
end
With the above code ("inspired" by the Nested Model Form Part 1 Rails Cast) my intent is to store category associations when creating an article (note: category objects are already present in the database; in my case, I would like just storing-creating category associations). However, when I submit the related form from the related view file, I get the following error (I am logging error messages):
{:"category_associations.article_id"=>["can't be blank"], :category_associations=>["is invalid"]}
Why it happens since validates_associated seems to run the method article.category_association.valid? but only if the article.category_association.article_id is not nil? How can I solve the problem with the presence validation of the article_id foreign key?
However, if I comment out the validates :article_id, :presence => true in the CategoryAssociation model class, it works as expected but it seems to be not a right approach to do not validate foreign keys.
If I comment out the validates_associated :category_associations in the Article model class, I still get the error:
{:"category_associations.article_id"=>["can't be blank"]}

Answer 1

Use inverse_of to link the associations and then validate the presence of the associated object, not the presence of the actual foreign key.
Example from the docs:
class Member < ActiveRecord::Base
has_many :posts, inverse_of: :member
accepts_nested_attributes_for :posts
end
class Post < ActiveRecord::Base
belongs_to :member, inverse_of: :posts
validates_presence_of :member
end

Answer 2

Since you have a possible nested form with accepts_nested_attributes_for, therefore in CategoryAssociation you need to make the validation conditional, requiring presence for only for only updates:
validates :article_id, presence: true, on: :update
Aside from Active Record associations, you should have a foreign key constraints at the db level.

Answer 3

If you're stucked with this kind of errors too, try to replace:
validates :article_id, :presence => true
validates :category_id, :presence => true
with:
validates :article, :presence => true
validates :category, :presence => true
worked for me.

Answer 4

Validations will run on create or save (as you'd expect), so ask yourself, "at each one of those is there a saved instance being referred to?", because without a save an instance won't have an id as it's the database that assigns the id.
Edit: Like I've said in the comments, if you're going to downvote then leave a comment as to why.