ValidationError when updating an instance of the Photo model in the admin form

[flebel]

ValidationError: [u'Select a valid choice. That choice is not one of the available choices.']

This exception is thrown when a model instance is being updated from the admin form. It is caused by the following line in the save method of the Photo model:
ExifTag.objects.create(key=key, value=metadata[key].human_value, photo=self)

I have yet to find a sane explanation. The ExifTag instances belonging to the photo are being deleted before they are recreated, but it doesn't fix it. For now, the unsatisfactory solution is that the EXIF tags are no longer updated when a photo is saved. This can be an issue right now as the actual JPEG files can be updated for a model instance, which will result in incoherent EXIF metadata for that file.

[flebel]

For now, commit 0e2c5c3 implements a fix that makes it impossible to update the file of an instance of the Photo model. Also, the EXIF tags are saved only when the Photo instance is initially created. This makes it impossible to recreate the use case that spawned the bug, therefore it is not the optimal solution.