title: Get_by_natural_key throwing AttributeError?
body: <p>I'm having trouble using Django's <code>get_by_natural_key()</code> method.  (Django 1.6)</p>
  
  <p>I have Items which use code numbers like this: <code>ABC_1234</code>.</p>
  
  <p>Manager and Model:</p>
  
  <pre><code>class ItemManager(models.Manager):
      def get_by_natural_key(self, code_number_):
          return self.get(code_number=code_number_)
  
  class Item(models.Model):
      code_number = models.CharField('Code Number',
          max_length = 10, unique = True)
      def __unicode__(self):
          return self.code_number
      def natural_key(self):
          return (self.code_number,)
  </code></pre>
  
  <p>In my tests, I have this query...</p>
  
  <pre><code>Item.objects.get_by_natural_key(code_number)
  </code></pre>
  
  <p>... which throws this error:</p>
  
  <pre><code>AttributeError: 'Manager' object has no attribute 'get_by_natural_key'
  </code></pre>
  
  <p>I'd like to use this <code>code_number</code> as a natural key for related model fixtures, as well as a general purpose means of querying this model.</p>
  
  <p>What am I doing wrong?  Any suggestions?</p>