title: Object has no attribute _state
body: <p>I'm developing Django application, and I have following error</p>
  
  <pre><code>'Sheep' object has no attribute _state
  </code></pre>
  
  <p>My models are constructed like this</p>
  
  <pre><code>class Animal(models.Model):
      aul = models.ForeignKey(Aul)
      weight = models.IntegerField()
      quality = models.IntegerField()
      age = models.IntegerField()
  
      def __init__(self,aul):
          self.aul=aul
          self.weight=3
          self.quality=10
          self.age=0
  
      def __str__(self):
          return self.age
  
  
  class Sheep(Animal):
      wool = models.IntegerField()
  
      def __init__(self,aul):
          Animal.__init__(self,aul)
  </code></pre>
  
  <p>What I must do?</p>