Kansas City on Rails

1. The Model Layer
2. Controllers and Views

Part 1 of this series came out exactly 3 months and 3 days ago. Special thanks to a reader named Edward who prodded me to finally add the controllers and views to this.

Going beyond the model layer for nested comments introduces a new programming idiom: recursion. Some ruby developers may not be familiar with it – especially if your experience is mostly web-related, where the need doesn’t come up as often. Recursion in a nutshell is the act of a method calling itself. If you’ve seen Inception, The ability to have dreams within dreams within dreams means those dreams are recursive. If you haven’t seen the movie, think of russian matryoshka dolls. You won’t experience star-studded special effects with the dolls, but you’ll at least get the idea of recursion.

Unlike russian dolls or most of Leo’s recent work, recursion in software is potentially infinite. Practically speaking though, it’s more like the doll thing. After all, a system only has so many resources, and recursion is expensive in this regard – the method must copy itself in memory at each layer, local variables and all. On the plus side, they tend to be lightning fast compared to standard iteration using loops. And in our case, we’ll be hitting the database at each layer. We’ll ignore the dangers in our simple app, though.

Routing

Let’s start with our routing file:

# config/routes.rb
NestedComments::Application.routes.draw do
  resources :comments do
    resources :comments
  end

  resources :posts do
    resources :comments
  end
  
  root :to => 'posts#index'
end

Working backward, we’re making our Posts controller’s index action our default route. That’s just to get the app functional. Next comes something interesting: nesting our comments inside of our posts. Interesting, but boring. Finally, the main event: nesting our comments within our comments!

Before you get too excited and start pulling out your Nana’s childhood russian doll set for comparision, this isn’t true recursion. It’s well documented that nesting resources any more than two layers deep is painful and unnecessary, so think of this as the lamest russian doll ever.

Controllers

First, our Posts controller, which is less exciting:

# app/controllers/posts_controller.rb
class PostsController < ApplicationController
  def index
    @posts = Post.all
  end

  def show
    @post = Post.find(params[:id])
  end

  def new
    @post = Post.new
  end
  
  def create
    @post = Post.new(params[:post])
    
    if @post.save
      redirect_to posts_path, :notice => "Your post was created successfully."
    else
      render :action => :new
    end
  end
end

We’re setting up a pretty standard restful resource here, with a couple actions skipped for simplicity. Now the comments controller (get those dolls ready):

# app/controllers/comments_controller.rb
class CommentsController < ApplicationController
  before_filter :get_parent
  
  def new
    @comment = @parent.comments.build
  end

  def create
    @comment = @parent.comments.build(params[:comment])
    
    if @comment.save
      redirect_to post_path(@comment.post), :notice => 'Thank you for your comment!'
    else
      render :new
    end
  end

  protected
  
  def get_parent
    @parent = Post.find_by_id(params[:post_id]) if params[:post_id]
    @parent = Comment.find_by_id(params[:comment_id]) if params[:comment_id]
    
    redirect_to root_path unless defined?(@parent)
  end
end

It’s not much bigger, but there’s a lot going on here! First, since comments are nested, we have to look for a parent. We’re only creating comments in this example, so we only have those related actions. Comments will always be shown on a post page.

The really exciting part is after a successful comment creation. How do we redirect back to the post page? For all we know, this comment could buried down 12 layers of replies. All we really have access to so far is the parent of the object. This necessitates a new model method:

# exerpt from app/models/comment.rb
def post
  return @post if defined?(@post)
  @post = commentable.is_a?(Post) ? commentable : commentable.post
end

Recursive functions are often short and sweet for two reasons: they’re already complex by nature, and adding more code than necessary would make them unmanageable. Also, they’re getting a lot done in just a few lines. In this case, the second line is the key: if “commentable” (the parent object) is a post, return that. Otherwise, call this same method on the parent, which will in turn check if *it* is a Post, and so on.

I could have written it shorter, like this:

def post
  commentable.is_a?(Post) ? commentable : commentable.post
end

In fact, I did at first. But the extra code that checks and sets an instance variable is caching the result. This way, if we call the same method on an object more than once, it stores the result for future use. Remember, recursion can be expensive – especially when the database is involved.

Views

Finally, it’s view time, with one more bit of recursion for fun.

Or post views are standard scaffolding mostly, with the exception of the show view:

# app/views/posts/show.html.erb
<h1><%= @post.title %></h1>

<div class="body">
  <%= @post.body %>  
</div>

<h2>Comments</h2>

<p><%= link_to 'Add a Comment', new_post_comment_path(@post) %></p>

<ul class="comment_list">
  <%= render :partial => 'comments/comment', :collection => @post.comments %>
</ul>

Notice we have the partial app/views/comments/_comment.html.erb. We’re calling this for each of our post’s comments. Nothing too fancy here. Now, for the partial itself:

# app/views/comments/_comment.html.erb
<li class="comment">
  <h3><%= comment.title %></h3>

  <div class="body">
    <%= comment.body %>
  </div>
  
  <p><%= link_to 'Add a Reply', new_comment_comment_path(comment) %></p>
  
  <% unless comment.comments.empty? %>
    <ul class="comment_list">
      <%= render :partial => 'comments/comment', :collection => comment.comments %>
    </ul>
  <% end %>
</li>

This partial is recursive! The comments controller doesn’t have a show method, because we’re never going to view a comment by itself. Instead, the show-like code is in this partial, and at the end it checks to see if *this* comment has comments. If so, it calls the partial again on the whole collection. The end result is a nested, bulleted list of comments. This is not very sexy if you fire up the code yourself, but it’s a great starting point.

Summary

Hopefully this article as done a good job of explaining both recursion, and how to use it to achieve nested comments in your applications. If you’re new to recursion as a concept, haven’t seen Inception, didn’t inherit russian dolls from Nana or receive them as a snazzy graduation present, and my explanation somehow fell short, it’s a well documented programming idiom. There are tons of resources online, so take the time to learn this powerful tool, then learn not to overuse it :)

Please download the code and play with it if you want to learn more – the code is fully test-driven so you can see how that works, which is just as important.

On a final note, I’m tempted to do a follow-up article with ajax and some nicer formatting. Perhaps in 3 months and 3 days…

Tags: comments, nesting, polymorphism, posts, Rails, recursion, recursive, Ruby, ruby on rails

This entry was posted on January 26, 2011 at 10:30 am and is filed under Database, Rails, Testing. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.