Why doesn't this partial render with UJS / jQuery?

Question

I'm trying to use jQuery to create posts with a form in my Rails 3 project. Here's what I have:

look below for updated results

create.js.erb:

$("#posts").prepend("#{render(:partial => 'posts')}");

layouts/application.html.erb:

...
<script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jquery/1.4.2/jquery.min.js"&gt;&lt;/script&gt;
<%= javascript_include_tag 'rails' %>
<%= csrf_meta_tag %>
...

posts_controller.rb:

def create
  @post = Post.new(params[:post])
  respond_to do |format|
    if @post.save
      # ...
      format.js
    else
      # ...
    end
  end
end

When I submit the post form it puts text like #{render(:partial => 'posts')} where the post ajax should be.

EDIT: by changing create.js.erb to:

$("#posts").prepend("<%= render(:partial => 'posts')%>");

I get this error in the terminal when I submit the form:

Rendered posts/_posts.html.erb (13.6ms)
Rendered posts/create.js.erb (80.4ms)
Completed   in 1631ms

ActionView::Template::Error (You have a nil object when you didn't expect it!
You might have expected an instance of Array.
The error occurred while evaluating nil.each):
    1: <%- for post in @posts -%>
    2:   <div class="post">
    3:     <%= link_to post.title, post %>
    4:     <%- if post.name? -%> 
  app/views/posts/_posts.html.erb:1:in     `_app_views_posts__posts_html_erb___692646030_16112196__738457576'
  app/views/posts/create.js.erb:1:in `_app_views_posts_create_js_erb__431255654_16166988__591965598'
  app/controllers/posts_controller.rb:39:in `create'

Rendered /usr/local/lib/ruby/gems/1.9.1/gems/actionpack-3.0.0/lib/action_dispatch/middleware/templates/rescues/_trace.erb (10.6ms)
Rendered /usr/local/lib/ruby/gems/1.9.1/gems/actionpack-3.0.0/lib/action_dispatch/middleware/templates/rescues/_request_and_response.erb (54.3ms)
Rendered /usr/local/lib/ruby/gems/1.9.1/gems/actionpack-3.0.0/lib/action_dispatch/middleware/templates/rescues/template_error.erb within rescues/layout (102.1ms)

Answer 1

you are missing the erb tag:

$("#posts").prepend("<%=render(:partial => 'posts')%>");

It appears that you are trying to render the full set of posts again, and I assume that you really want to render a single post. This would result in duplicate posts showing up. No @posts variable was set in your create method resulting in that nil error. You should create another partial called _post and render a single post within there. Notice that the create method sets @post, but not @posts. That is where your error is coming from, because @posts is nil. The error is a bit misleading b/c you are using:

for post in @posts

I would recommend using the following syntax that is more widely using in Ruby (which has the same effect as "for"):

@posts.each do |post|

I think this railscast may help you. Based on what you are showing here, I think Ryan is doing almost the exact same thing, but using a polling method instead of submitting a form. Pay special attention to the part where he creates _comment.html.erb to render a single comment.

http://railscasts.com/episodes/229-polling-for-changes