Trouble with accepts_nested_attributes_for on validating foreign key

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.

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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