Blank page when rendering a partial with ajax in rails 3 - ruby-on-rails

Question

I have a nav menu with 2 tabs/links in the show.html.erb file, in UsersController.rb, I would like to use ajax to render different partial for the tabs.
In the show.html.erb I have a div named profile-data where I want to show the content.
So I do something like this:
The link structure:
<li><%= link_to "College friends", college_friends_path, :remote => true %></li>
<li><%= link_to "Highschool friends", highschool_friends_path, :remote => true %></li>
I define the routes:
match "college_friends" => "users#college_friends", :as => "college_friends"
match "highschool_friends" => "users#highschool_friends, :as => "highschool_friends"
And I define in my UserController.rb the necessary methods:
class UsersController < ApplicationController
def show
#user = User.find(params[:id])
end
def college_friends
respond_to do |format|
format.js
end
end
def highschool_friends
respond_to do |format|
format.js
end
end
end
Last thing we have the JS files:
*college_friends.js.erb*
$('#profile-data').html("<%= escape_javascript(render(:partial => 'college_friends')) %>");
*highschool_friends.js.erb*
$('#profile-data').html("<%= escape_javascript(render(:partial => 'highschool_friends')) %>");
The partial code: _college_friends.html.erb
<% groups = #user.friends.group_by(&:college_name) %>
<% sorted_groups = groups.sort_by{|key, values| values.count}.reverse %>
<% sorted_groups.each do |collegename, friends| %>
<% next if collegename.blank? %>
<div class="contentbox">
<div class="box-header">
<h3><%= collegename %></h3>
<div class="meta-info">
<p><i class="icon-map-marker"></i> Malmö</p>
<p><i class="icon-user"></i><span class="count"> <%= friends.count %></span> vänner</p>
</div>
</div>
<ul class="friends-list">
<% friends.map do |friend| %>
<li><%= image_tag(friend.image) %>
<% end %>
</ul>
</div>
<% end %>
So I solve this problem but checking the source in and out. And I notice that the js files was not loaded in the app.
So I changed this:
<%= javascript_include_tag :defaults %>
To this:
<%= javascript_include_tag "application" %>
And it loaded all the necessary js files.
But then I get this error:
ActionView::Template::Error (undefined method `friends' for nil:NilClass):

Answer 1

You are getting
ActionView::Template::Error (undefined method `friends' for nil:NilClass):
Because
<% groups = #user.friends.group_by(&:college_name) %>
requires #user to be set.
As Brandon suggested, try
def college_friends
#user = User.find(params[:id])
respond_to do |format|
format.js
end
end
BUT
To achieve this, you need to change your link routes to include user id, similar to this
match "college_friends/:id" => "users#college_friends", :as => "college_friends"
and in your links
<li><%= link_to "College friends", college_friends_path(#user), :remote => true %></li>

Answer 2

As suggested in the comments above, you should have two partials named _college_friends.html.erb and _highschool_friends.html.erb
These will contain your HTML, which is what you're wanting to load via $('#profile-data').html()
Template Error:
You are not defining #user in college_friends and highschool_friends. So your view is requesting #user from that action, which isn't there.