Ajax update doesn't function with Rails

I have a _follow_form partial :

 <% unless current_user == @player %>
  <div id="follow_form">
      <% if current_user.following?(@player) %>
        <%= render 'unfollow' %>
      <% else %>
        <%= render 'follow' %>
      <% end %>
  </div>
<% end %>

My _follow partial looks like this :

<%= form_for current_user.relationships.build(:followed_id => @player.id),
             :remote => true do |f| %>
  <div><%= f.hidden_field :followed_id %></div>
  <div class="actions"><%= f.submit "Follow" %></div>
<% end %>

My create.js.erb file :

$("#follow_form").update("<%= escape_javascript(render('players/follow')) %>")

But nothing happens when I create a Relationship. I mean the relation is created, but the partial doesn't update. When I try with :

$("#follow_form").append("foobar");

It works.

How could I update #follow_form ? Thanks for your help.

PS: I searched for a solution before posting, but everything I tried failed.

UPDATE

With :

$("#follow_form").html("<%= escape_javascript(render('players/unfollow')).html_safe %>")

Nothing happens, the partial isn't replaced but an error occurs :

NoMethodError in Relationships#create

Showing C:/Ruby/ostriker/app/views/players/_unfollow.html.erb where line #1 raised:

undefined method `model_name' for NilClass:Class

My _unfollow partial :

<%= form_for current_user.relationships.find_by_followed_id(@player),
         :html => { :method => :delete },
         :remote => true do |f| %>
 <div class="actions"><%= f.submit "Unfollow" %></div>
<% end %>