title: dependent: :destroy doesn't work (has_many association)
body: <p>My problem is as simple as is the title of this thread
  i have two classes</p>
  
  <pre><code>class Category &lt; ActiveRecord::Base
  
    has_many :subcategories, dependent: :destroy
  
    accepts_nested_attributes_for :subcategories
  
  end
  
  class Subcategory &lt; ActiveRecord::Base
    belongs_to :category
  end
  </code></pre>
  
  <p>but when i hit <code>link_to "Delete", category_path(category), method: :delete</code> requested category gets destroyed, but all subcategories stay like nothing happened.</p>
  
  <p>EDIT</p>
  
  <p>Server Log:</p>
  
  <pre><code>Started DELETE "/categories/8" for 127.0.0.1 at 2013-10-29 01:38:01 +0100
  Processing by CategoriesController#destroy as HTML
    Parameters: {"authenticity_token"=&gt;"rOTo3ROv/QsQRMPyCljbMvrViKgihC1CowBdlHdq7O4=", "id"=&gt;"8"}
    Category Load (0.3ms)  SELECT `categories`.* FROM `categories` WHERE `categories`.`id` = 8 LIMIT 1
    SQL (0.5ms)  DELETE FROM `categories` WHERE `categories`.`id` = 8
  Redirected to http://localhost:3000/categories
  Completed 302 Found in 3ms (ActiveRecord: 0.8ms)
  
  
  Started GET "/categories" for 127.0.0.1 at 2013-10-29 01:38:01 +0100
  Processing by CategoriesController#index as HTML
    Category Load (0.3ms)  SELECT `categories`.* FROM `categories`
    Rendered categories/index.html.haml within layouts/application (0.9ms)
    Rendered application/_header.html.haml (0.1ms)
  Completed 200 OK in 6ms (Views: 5.5ms | ActiveRecord: 0.3ms)
  </code></pre>