Write your forwards methods here.

def forwards(self, orm):
        "Write your forwards methods here."
        # Note: Don't use "from appname.models import ModelName". 
        # Use orm.ModelName to refer to models in this application,
        # and orm['appname.ModelName'] for models in other applications.
        from account_keeping.models import Invoice
        for invoice in Invoice.objects.all():
            invoice.save()