title: Rails 3 finding parents which have no child
body: <p>In a one to many relationship with no counter cache how can I find parents with no child?</p>
  
  <p>user.rb</p>
  
  <pre><code>has_many :pages
  </code></pre>
  
  <p>page.rb</p>
  
  <pre><code>belongs_to :user
  </code></pre>
  
  <p>I've tried</p>
  
  <pre><code>User.includes(:pages).where("pages.user_id is NULL")
  </code></pre>
  
  <p>This is making trouble in MySQL.</p>