has_one, :through => model VS simple method?

Question

I have some issues using has_one, through => model. The best is to show you my case.

class Category
  has_many :articles
end

class Article
  has_many :comments
  belongs_to :category
end

class Comment
  belongs_to :article
  has_one :category, :through => :articles
end

Everthing works fine. I can do comment.category. The problem is when I create a new comment and set up its article, I have so save the comment to make the association works. Example :

 >> comment = Comment.new
 >> comment.article = Article.last
 >> comment.category
     -> nil
 >> comment.article.category
     -> the category
 >> comment.save
 >> comment.category
     -> nil
 >> comment.reload
 >> comment.category
     -> the category

has_one, through => model anyway do not set up, build constructor and create method. So, I want to replace my comment model by :

class Comment
  belongs_to :article
  def category
    article.category
  end
end

Sounds a good idea ?

Answer 1

Nothing wrong with your idea. I can't see many situations in which has_one :category, :through => :articles would be the obvious better choice (unless eager-loading with Comment.all(:include => :category) ).

A hint on delegate:

class Comment
  belongs_to :article
  delegate :category, :to => :article

A different approach:

class Comment
  belongs_to :article
  has_one :category, :through => :article

  def category_with_delegation
    new_record? ? article.try(:category) : category_without_delegation
  end

  alias_method_chain :category, :delegation