title: Override django form's clean method to change the ModelChoiceField queryset
body: <p>I have this model:</p>
  
  <pre><code>class PetAttribute(models.Model):
      species = models.ForeignKey(PetSpecies, related_name = "pets")
      pet_category = models.ForeignKey(PetCategory, related_name = "pets", blank=True, null=True)
      pet_type = models.ForeignKey(PetType, related_name = "pets", blank=True, null=True)
      additional_fields= models.ManyToManyField( AdditionalField, null= True, blank=True )
  </code></pre>
  
  <p>Now i want to add an additional option in select (pet_category, pet_type), which is </p>
  
  <blockquote>
    <p>("0","Other")</p>
  </blockquote>
  
  <p>in queryset of these. I have tried but form give me an error </p>
  
  <blockquote>
    <p>Error: Select a valid choice. That choice is not one of the available choices.</p>
  </blockquote>
  
  <p><a href="https://stackoverflow.com/questions/5281195/forms-modelchoicefield-queryset-extra-choice-fields-django-forms">here</a> is a one solution of it, but i want to do this by <strong>ModelChoiceField</strong> </p>
  
  <p>Any suggestion?</p>
  
  <p>Thanks :)</p>