I assume you're using Django 1.6. If so you can do the following according to the docs.
from django.forms import ModelChoiceField
class ProviderModelChoiceField(ModelChoiceField):
def label_from_instance(self, obj):
return obj.slug
Then you need to use the ProviderModelChoiceField
in your form.
Otherwise if you're using 1.7, you can use the argument to_field_name
. Here are the docs.