code: def contribute_to_class(self, cls, name, **kwargs):
          
          super(PlaceholderField, self).contribute_to_class(cls, name, **kwargs)
  
          # overwrites what instance.<colname> returns; give direct access to the placeholder
          setattr(cls, name, PlaceholderFieldDescriptor(self.slot))
  
          # Make placeholder fields easy to find
          # Can't assign this to cls._meta because that gets overwritten by every level of model inheritance.
          if not hasattr(cls, '_meta_placeholder_fields'):
              cls._meta_placeholder_fields = {}
          cls._meta_placeholder_fields[name] = self
  
          # Configure the revere relation if possible.
          # TODO: make sure reverse queries work properly
          if django.VERSION >= (1, 11):
              rel = self.remote_field
          else:
              rel = self.rel
  
          if rel.related_name is None:
              # Make unique for model (multiple models can use same slotnane)
              rel.related_name = '{app}_{model}_{slot}_FIXME'.format(
                  app=cls._meta.app_label,
                  model=cls._meta.object_name.lower(),
                  slot=self.slot
              )
  
              # Remove attribute must exist for the delete page. Currently it's not actively used.
              # The regular ForeignKey assigns a ForeignRelatedObjectsDescriptor to it for example.
              # In this case, the PlaceholderRelation is already the reverse relation.
              # Being able to move forward from the Placeholder to the derived models does not have that much value.
              setattr(rel.to, self.rel.related_name, None)
docs: Internal Django method to associate the field with the Model; it assigns the descriptor.