Adding dynamic methods to ModelAdmin fails in 1.8

Hi,

I'm trying to add a dynamic method to a ModelAdmin so that I can have automatically generated thumbnail fields for any image by simply adding '<fieldname>_thumbnail' to the field definitions - which saves a lot of code when there's a load of admin classes with image fields that need thumbnails.

e.g.

class ThumbnailMixin(object):

    def getattr(self, name):

         if name.endswith('_thumbnail'):

              # ... generate appropriate thumbnail method and return

              return thumbnail_function

         else:

             raise AttributeError

class ExampleAdmin(models.ModelAdmin, ThumbnailMixin):

     fields = ['image_thumbnail', 'image', ...]

This worked fine in Django 1.6/1.7, but it's now failing in 1.8 with a SystemCheckError (admin.E035 / admin.E108). If I silence the error, the admin actually works fine, but that feels icky.

The check fails because it checks for the field being on the ExampleAdmin class, not on an instance - whereas the admin always uses an instance, so it works.

What's the "right" way to do this in the 1.8 world - I've considered a custom metaclass, but that feels like overkill for a simple task like this. Should this even work, and if so is it a bug in the check framework?

Cheers,

Malcolm

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