django - "Cannot filter a query once a slice has been taken"

Question

I get this error

Caught AssertionError while rendering: Cannot filter a query once a slice has been taken.

On this line

{% if form.non_field_errors %} 

When I try to do this

copy_pickup_address = ModelChoiceField(required=False, queryset=Address.objects.filter(shipment_pickup__user=user).order_by('-shipment_pickup__created')[:5])

But I need to slice it, because I only want the last 5 addresses. It renders fine, until I choose an address and submit the form. Why doesn't it like this? How can I get around it?

Answer 1

The explanation is given at https://docs.djangoproject.com/en/1.8/ref/models/querysets/:

even though slicing an unevaluated QuerySet returns another unevaluated QuerySet, modifying it further (e.g., adding more filters, or modifying ordering) is not allowed, since that does not translate well into SQL and it would not have a clear meaning either.