Question

As the official site, I defined two models Post and Comment. Comment is nested in Post.

resources :posts do
  resources :comments
end

So I can use @post.comments to access all comments a post own. In console:

$ post = Post.new()
$ post.comments.class
> Array

But:

$ post.comments.respond_to?("build")
> true

Why? An array has the method build? How did Rails do? And how can I know other hidden methods of post.comments?

Answer 1

Firstly, your resources calls are in your routes.rb file, which only deals with the URL parsing side of Rails. It’s nothing to do with the associations between your models, which are set up using has_many and belongs_to calls in the relevant model files. Ignore the routes file for now as it’s not related to the main part of your question.

With respect to associations, you’ll find that post.comments is not returning you an Array, it’s actually returning an ActiveRecord::Relation object. A Relation object is like an array – in fact any method you call on it which isn’t relation-specific (like build) is actually passed on to an Array representation of the Relation‘s contents. So, when you call post.comments.length, the comments association object is calling .length on its internal array.

One of the consequences of this is that calling post.comments.class actually passes on the .class call to the Array too!

So, what can that Relation object actually do? Since the association is set up by the has_many call, you can find out in the has_many documentation. Similarly for a belongs_to association