title: Django admin choice field dynamically populated by generic foreign key's model fields
body: <p>Say I have the following simple models for some tagging application (this is simplified from the actual code):</p>
  
  <pre><code># Model of tag templates
  class TagTemplate(models.Model):
      name = models.CharField()
      content_type = models.ForeignKey(ContentType)
  
  class Tag(models.Model):
      template = models.ForeignKey(TagTemplate)
      object_id = models.PositiveIntegerField()
   *  content_object = generic.GenericForeignKey('template__content_type', 'object_id') 
  
  # Each tag may display the 
  class TagTemplateItemDisplay(models.Model):
      template = models.ForeignKey(TagTemplate)
      content_type_field = models.CharField()
      font_size = models.IntegerField()
  </code></pre>
  
  <p>I have two questions:</p>
  
  <p>1) In the line marked with the *, I understand from the documentation that I need to pass the two field names as per the contenttype framework. In my case the content_type field is specified within the template model. I'd like to avoind a duplicate content_type field within the 'tag' model to get the GenericForeignKey working. Is this possible? Or do I need some custom manager to implement a duplicate content_type within the tag model?</p>
  
  <p>2) I'd like to use the admin site with these models. Is it possible to dynamically create a choice dropdown for the 'content_type_field' field where the contents corresponds to a list of fields from the chosen content_type of the parent model (ie. tagTemplate) when using Tabularinline layout?</p>
  
  <p>eg. in the admin site I pick a model (content_type field) for a new tagTemplate record that contains the fields ('name', 'age', 'dob'), I'd like the TabularInline forms to dynamically update the 'content_type_field' to contain the choices name, age and dob. If i then pick a different model in the parent tagTemplate content_type field, the choices in the child tagTemplateItemDisplay content_type_field of the inline are updated again.</p>