Hi Chris How do I add jquery and ajax to the comments form from your tutorial?

I wrote some jquery to accomplish this here is the code.

$(window).load(function(e){

$("#comment_body").on('click',function(){
$(this).removeAttr('placeholder');

});

var comment = $("#comment_body").val();
var id = '#{params[:video_id]}';
$("#comment_body").keypress(function(event) {
if (event.which == 13) {
var request = $.ajax({
type:"POST" ,
url: '/videos/comments' ,
data:{ "comment":$(this).val()},
timeout : 5000

  }); 
  request.success(function() {
          $("#error-information").html("comment has been posted");
        });      
  request.error(function(httpObj, textStatus) {       
            if(httpObj.status==401){
                window.location ="http://localhost:3000/users/sign_in";
             }
        });
  // Clear out the form so it can be used again    
   $('#comment_body').val('').blur();
   $("#comment_body").attr('placeholder', 'Join The discussion...');       
}    

});

});

But I keep getting this error from the comments controller?

NoMethodError in CommentsController#create

undefined method `comments' for nil:NilClass
at this line @comment = @commentable.comments.new comment_params
Now can someone tell me exactly what I am doing wrong here?