Not working query_set.query.group_by = [..] in v1.8

I am upgrade from Django v 1.6 to v1.8. I use this code:

query_set = Rubrica.objects.filter(...).annotate(count=Count('..')).order_by('user', '-pub_date')

query_set.query.group_by = ['user_id']

On Django version 1.6 this is works fine, but on version 1.8 results is not grouped by user_id field.

How can I solve this problem?

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