Widget formatting for foreign key relations doesn't work

[geeknam]

Assuming Author model has birthday field. The following widget formatting wouldn't work.

        class BookResource(resources.ModelResource):

            class Meta:
                model = Book
                fields = ('author__birthday',)
                widgets = {
                    'author__birthday': {'format': '%Y-%m-%d'},
                    # However this works
                    'birthday':  {'format': '%Y-%m-%d'},
                }