Question

I’m reading Beginning Rails 3. It creates a blog with Users who can post Articles and also post Comments to these Articles. They look like this:

    class User < ActiveRecord::Base
      attr_accessible :email, :password, :password_confirmation
      attr_accessor :password

      has_many :articles, :order => 'published_at DESC, title ASC',
                          :dependent => :nullify
      has_many :replies, :through => :articles, :source => :comments

    class Article < ActiveRecord::Base
      attr_accessible :body, :excerpt, :location, :published_at, :title, :category_ids

      belongs_to :user
      has_many :comments

    class Comment < ActiveRecord::Base
      attr_accessible :article_id, :body, :email, :name
      belongs_to :article

in app/views/comments/new.html.erb there’s a form which begins like this:

    <%= form_for([@article, @article.comments.new]) do |f| %>

My confusion lies in why form_for() has two parameters. What do they resolve to and why are they necessary?

thanks,
mike

Answer 1

Actually, in your example, you are calling form_forwith one parameter (which is Array). If you check the documentation you will see parameters it expects: form_for(record, options = {}, &proc).
In this case a record can be ActiveRecord object, or an Array (it can be also String, Symbol, or object that quacks like ActiveRecord). And when do you need to pass it an Array?

The simplest answer is, when you have a nested resource. Like in your example, you have defined Article has many Comments association. When you call rake routes, and have correctly defined routes, you will see that Rails has defined for you different routes for your nested resource, like: article_comments POST /article/:id/comments.

This is important, because you have to create valid URI for your form tag (well not you, Rails does it for you). For example:

form_for([@article, @comments])

What you are saying to Rails is: “Hey Rails, I am giving you Array of objects as a first parameter, because you need to know the URI for this nested resource. I want to create new comment in this form, so I will give you just initial instance of @comment = Comment.new. And please create this comment for this very article: @article = Article.find(:id).”

This is roughly similar to writing:

form_for(@comments, {:url => article_comments_path(@aticle.id)})

Of course, there is more to the story, but it should be enough, to grasp the idea.