How can you validate the presence of a belongs to association with Rails?
ruby, ruby-on-rails
Solution
I've also been investigating this topic and here is my summary:
The root cause why this doesn't work OOTB (at least when using `validates_presence_of :article` and not `validates_presence_of :article_id`) is the fact that rails doesn't use an identity map internally and therefore will not by itself know that `article.comments[x].article == article`
I have found three workarounds to make it work with a little effort:
- Save the article before creating the comments (rails will automatically pass the article id that was generated during the save to each newly created comments; see Nicholas Hubbard's response)
- Explicitly set the article on the comment after creating it (see W. Andrew Loe III's response)
- Use inverse_of:
class Article < ActiveRecord::Base
has_many :comments, :inverse_of => :article
end
`
This last solution was bot yet mentioned in this article but seems to be rails' quick fix solution for the lack of an identity map. It also looks the least intrusive one of the three to me.
Problem
Say I have a basic Rails app with a basic one-to-many relationship where each comment belongs to an article: ``` $ rails blog $ cd blog $ script/generate model article name:string $ script/generate model comment article:belongs_to body:text ``` Now I add in the code to create the associations, but I also want to be sure that when I create a comment, it always has an article: ``` class Article < ActiveRecord::Base has_many :comments end class Comment < ActiveRecord::Base belongs_to :article validates_presence_of :article_id end ``` So now let's say I'd like to create an article with a comment all at once: ``` $ rake db:migrate $ script/console ``` If you do this: ``` >> article = Article.new => #<Article id: nil, name: nil, created_at: nil, updated_at: nil> >> article.comments.build => #<Comment id: nil, article_id: nil, body: nil, created_at: nil, updated_at: nil> >> article.save! ``` You'll get this error: ``` ActiveRecord::RecordInvalid: Validation failed: Comments is invalid ``` Which makes sense, because the comment has no page_id yet. ``` >> article.comments.first.errors.on(:article_id) => "can't be blank" ``` So if I remove the `validates_presence_of :article_id` from `comment.rb`, then I could do the save, but that would also allow you to create comments without an article id. What's the typical way of handling this? UPDATE: Based on Nicholas' suggestion, here's a implementation of save_with_comments that works but is ugly: ``` def save_with_comments save_with_comments! rescue false end def save_with_comments! transaction do comments = self.comments.dup self.comments = [] save! comments.each do |c| c.article = self c.save! end end true end ``` Not sure I want add something like this for every one-to-many association. Andy is probably correct in that is just best to avoid trying to do a cascading save and use the nested attributes solution. I'll leave this open for a while to see if anyone has any other suggestions.