how to make certain that the queryset will be ordered by a valid field - django

Question

This is a portion of a template tag code, where qs is a queryset.

def foo(qs):
    ...
    context['key'] = qs.order_by('an_invalid_field_coming_from_user')

How can I check if the queryset will be ordered by a valid field before the code execution goes out of the scope of the template tag, other than forcing an evaluation?

The code as is does not raise an error since the queryset is not evaluated. qs.exists() is not the answer either as it will execute the query without being ordered.

EDIT: Please note that the query may be more complex than my pre-edit simple example Foo.objects.all(), eg, it may feature an extra() method which results in joins.

Answer 1

Using order_by() with a field that doesn't exist will throw a django.core.exceptions.FieldError. Simply catch that and report the error to the user.

Answer 2

I am not sure that's a good idea as there might be security considerations here. I think this will work but I don't have a copy of Django around to test for sure that the moment. I am pretty sure there is an exception thrown when you do that but if you don't want to catch that or check preemptively this should work:

if hasattr(Foo, "fieldname") and isinstance(getattr(Foo, "fieldname"), models.Field):
    print "Safe field"
else:
    print "Invalid field"

Answer 3

If you're really against catching an exception (which you shouldn't be), you can do this:

if context['key'] in [field.name for field in Foo._meta.fields]:
    qs = Foo.objects.all().order_by(context['key'])