title: validates_associated seems not working
body: <p>I want to ask you a question about the helper validates_associated, since I need to validate a asocciacion. When I try save a new book without associate this a one library the validation does not work</p>
  
  <p>This is my schema.rb</p>
  
  <p>ActiveRecord::Schema.define(version: 20140806165743) do</p>
  
  <pre><code>  create_table "books", force: true do |t|
      t.string   "title"
      t.integer  "price"
      t.integer  "library_id"
      t.datetime "created_at"
      t.datetime "updated_at"
    end
  
    create_table "libraries", force: true do |t|
      t.string   "name"
      t.datetime "created_at"
      t.datetime "updated_at"
    end     
  end
  </code></pre>
  
  <p>and these are my two models</p>
  
  <pre><code>class Library &lt; ActiveRecord::Base
   has_many :books 
   validates_associated :books
  end
  
  class Book &lt; ActiveRecord::Base
   belongs_to :library
  end
  </code></pre>
  
  <p>Rails Console</p>
  
  <pre><code>library=Library.new(name: "Rayuela")
   =&gt; #&lt;Library id: nil, name: "Rayuela", created_at: nil, updated_at: nil&gt; 
  2.0.0-p481 :003 &gt; library.save
     (0.2ms)  SAVEPOINT active_record_1
    SQL (3.6ms)  INSERT INTO "libraries" ("created_at", "name", "updated_at") VALUES (?, ?, ?)  [["created_at", Wed, 06 Aug 2014 21:50:56 UTC +00:00], ["name", "Rayuela"], ["updated_at", Wed, 06 Aug 2014 21:50:56 UTC +00:00]]
     (0.1ms)  RELEASE SAVEPOINT active_record_1
   =&gt; true 
  2.0.0-p481 :004 &gt; book1=Book.new(title:"Ali Baba",price:50)
   =&gt; #&lt;Book id: nil, title: "Ali Baba", price: 50, library_id: nil, created_at: nil, updated_at: nil&gt; 
  2.0.0-p481 :005 &gt; book1.save
     (0.2ms)  SAVEPOINT active_record_1
    SQL (0.8ms)  INSERT INTO "books" ("created_at", "price", "title", "updated_at") VALUES (?, ?, ?, ?)  [["created_at", Wed, 06 Aug 2014 21:51:46 UTC +00:00], ["price", 50], ["title", "Ali Baba"], ["updated_at", Wed, 06 Aug 2014 21:51:46 UTC +00:00]]
     (0.1ms)  RELEASE SAVEPOINT active_record_1
   =&gt; true 
  </code></pre>
  
  <p>The book1 was saved in the database without to be associated to a library
  It has got library_id=nil??????????????</p>