title: Default value for hidden field in Django model
body: <p>I have this Model:</p>
  
  <pre><code>class Occurrence(models.Model):
      id = models.AutoField(primary_key=True, null=True)
      reference = models.IntegerField(null=True, editable=False)
  
      def save(self):
           self.reference = self.id
           super(Occurrence, self).save()
  </code></pre>
  
  <p>I want for the reference field to be hidden and at the same time have the same value as id. This code works if the <code>editable=True</code> but if I want to hide it it doesn't change the value of reference. </p>
  
  <p>How can I fix that?</p>