title: NoMethodError in UsersController#update
body: <p><strong>The error:</strong></p>
  
  <pre><code>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.[]
  
  
  app/controllers/users_controller.rb:45:in `update'
  </code></pre>
  
  <p><strong>Controller users_controller</strong></p>
  
  <pre><code>def update
      @user.groups = Group.find(@params[:group_ids]) if @params[:group_ids]
      if @user.update_attributes(params[:user])
        flash[:success] = "User updated."
        #Redirect back to current users, i've change this as after the admin edit a user it would redirect them
        #Too that users account which conflicts.
        #if current_user.admin? # If the current user is an admin move them back to the user list page.
          #redirect_to users_path
        #else # if its a normal user just redirect them back to there dashboard
          redirect_to @user
        #end
      else
        @title = "Edit user"
        render 'edit'
      end
    end
  </code></pre>
  
  <p><strong>The view (edit.html.erb):</strong></p>
  
  <pre><code>&lt;% for group in @groups %&gt;
      &lt;%= check_box_tag "user[group_ids][]", group.id %&gt;
      &lt;%= group.description %&gt;
      &lt;% end %&gt;
  </code></pre>