Need help on a (maybe complex) query with aggregate/annotate

I'm having a bit of hard time with aggregation.

For simplicity's sake consider the models available in the aggregation
docs:
https://docs.djangoproject.com/en/1.3/topics/db/aggregation/#

This query:
Publisher.objects.filter(book__rating__gt=3.0).annotate(num_books=Count('book'))
returns all the publishers with at least one good book (ranked > 3)
with annotated the number of good books for each publisher.

How can i modify the query to get the list of ALL publishers with
annotated the number of good books for each publisher?
In other words I want to keep in the results also the Publishers
without good books (num_books = 0).

Ciao
Enrico

PS: Sorry for my horrible english

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