Rails 3 - JSON of nested models not returning as expected

I'm trying to set up a javascript front in that is consuming JSON from a Rails backend.

The only problem is, when I have nested models in Rails, I'm not sure if the JSON being returned is in the correct format.

I have a Post model and a Comment model. A Post has many Comments. Each model has a 'title' and 'content' field.

Here's where my problem is so far...

1) When I visit the url: http://localhost:3000/posts.json I get the expected output:

> [{"content":"My first
> post","created_at":"2012-02-07T18:56:16Z","id":1,"title":"Hello","updated_at":"2012-02-07T18:56:16Z"},{"content":"More
> crap","created_at":"2012-02-07T21:30:51Z","id":2,"title":"My 2nd
> Post","updated_at":"2012-02-07T21:30:51Z"}]

2) If I put in this url: http://localhost:3000/posts/1/comments/4.json I also get the expected:

> {"content":"that's
> nice","created_at":"2012-02-07T20:57:16Z","id":4,"post_id":1,"title":"cool","updated_at":"2012-02-07T20:57:16Z"}

3) But... if I put in a url like this: http://localhost:3000/posts/1/comments.json

it returns null.

I don't have access to the JSON version of the list of Comments associated with the Post.

Using the standard Rails views, I can do full CRUD with the nesting no problem, but do I need to do something different from the way Rails normally passes the JSON back in order to consume it on the client side?

EDIT

I'm assuming you may need to see the controller code for the comments controller. Here it is:

class CommentsController < ApplicationController

  before_filter :get_post  

  respond_to :html, :xml, :json

  def index
    @comments = @post.comments.all    
    respond_with (@comment)    
  end

  def show
    @comment = @post.comments.find(params[:id])

    respond_with(@comment)

  end

  def new
    @comment = @post.comments.build

    respond_with(@comment)
  end

  def edit
    @comment = @post.comments.find(params[:id])
  end

  def create
    @comment = @post.comments.build(params[:comment])

    respond_to do |format|
      if @comment.save
        format.html { redirect_to([@post, @comment], :notice => 'Comment was successfully created.') }
        format.xml  { render :xml => @comment, :status => :created, :location => @comment }
        format.json  { render :json => @comment, :status => :created, :location => @comment }
      else
        format.html { render :action => "new" }
        format.xml  { render :xml => @comment.errors, :status => :unprocessable_entity }
        format.json  { render :json => @comment.errors, :status => :unprocessable_entity }
      end
    end
  end

 def update
    @comment = @post.comments.find(params[:id])

    respond_to do |format|
      if @comment.update_attributes(params[:comment])
        format.html { redirect_to(@comment, :notice => 'Comment was successfully updated.') }
        format.xml  { head :ok }
        format.json  { head :ok }
      else
        format.html { render :action => "edit" }
        format.xml  { render :xml => @comment.errors, :status => :unprocessable_entity }
        format.json  { render :json => @comment.errors, :status => :unprocessable_entity }
      end
    end
  end

  def destroy
    @comment = @post.comments.find(params[:id])
    @comment.destroy

    respond_to do |format|
      format.html { redirect_to(post_comments_url) }
      format.xml  { head :ok }
      format.json  { head :ok }
    end
  end  

  protected  

  def get_post
    @post = Post.find_by_id(params[:post_id])
    redirect_to root_path unless @post
  end

end