code: def register(model, field_name=None, related_name=None, lookup_method_name='get_follows'):
      
      if model in registry:
          return
  
      registry.append(model)
      
      if not field_name:
          field_name = 'target_%s' % model._meta.module_name
      
      if not related_name:
          related_name = 'follow_%s' % model._meta.module_name
      
      field = ForeignKey(model, related_name=related_name, null=True,
          blank=True, db_index=True)
      
      field.contribute_to_class(Follow, field_name)
      setattr(model, lookup_method_name, get_followers_for_object)
      model_map[model] = [related_name, field_name]
docs: This registers any model class to be follow-able.