how to use ajax with json in ruby on rails

Question

I am implemeting a facebook application in rails using facebooker plugin, therefore it is very important to use this architecture if i want to update multiple DOM in my page. if my code works in a regular rails application it would work in my facebook application.

i am trying to use ajax to let the user know that the comment was sent, and update the comments bloc.

migration:

class CreateComments < ActiveRecord::Migration
 def self.up
    create_table :comments do |t|
      t.string :body

      t.timestamps
    end
  end

  def self.down
    drop_table :comments
  end
end

controller:

class CommentsController < ApplicationController

  def index
    @comments=Comment.all
  end

 def create
@comment=Comment.create(params[:comment])

  if request.xhr?
      @comments=Comment.all
      render :json=>{:ids_to_update=>[:all_comments,:form_message],
               :all_comments=>render_to_string(:partial=>"comments" ),
               :form_message=>"Your comment has been added." }
  else
    redirect_to comments_url
  end
 end

end

view:

<script>
function update_count(str,message_id) {
len=str.length;
if (len < 200) {
  $(message_id).innerHTML="<span style='color: green'>"+
    (200-len)+" remaining</span>";
} else {
  $(message_id).innerHTML="<span style='color: red'>"+
    "Comment too long. Only 200 characters allowed.</span>";
}
}


function update_multiple(json) {
for( var i=0; i<json["ids_to_update"].length; i++ ) {
    id=json["ids_to_update"][i];
    $(id).innerHTML=json[id];
}
}

</script>

<div id="all_comments" >
 <%= render :partial=>"comments/comments" %>
</div>

Talk some trash: <br />
<% remote_form_for Comment.new,
  :url=>comments_url,
  :success=>"update_multiple(request)" do |f|%>
<%= f.text_area :body,
 :onchange=>"update_count(this.getValue(),'remaining');" ,
 :onkeyup=>"update_count(this.getValue(),'remaining');"
%> <br />
<%= f.submit 'Post'%>
<% end %>

<p id="remaining" >&nbsp;</p>
<p id="form_message" >&nbsp;</p>
<br><br>
<br>

if i try to do alert(json) in the first line of the update_multiple function , i got an [object Object].

if i try to do alert(json["ids_to_update"][0]) in the first line of the update_multiple function , there is no dialog box displayed.

however the comment got saved but nothing is updated.

it seems like the object sent by rails is nil or cant be parsed by JSON.parse(json).

questions:

1.how can javascript and rails know that i am dealing with json objects?deos ROR sent it a object format or a text format?how can it check that the json object has been sent

2.how can i see what is the returned json?do i have to parse it?how?

2.how can i debug this problem?

3.how can i get it to work?