Annotating a queryset without aggregation

Hi all

I want to annotate a qs with a computed field from one of its current
fields, and then filter on the annotated field. I cannot work out how
I'm supposed to do this in the ORM (or even if I can).

Basically, I have a model with a username field, that in this case
contains an email address. I want to extract the domain of that email
address, and then filter the queryset based upon the value of the
domain.

So, in code:

domains = UsageLogEntry.objects \
.filter(action=ACTION_FAILED_REGISTRATION,
sub_action=SUB_ACTION_DOMAIN_DISALLOWED) \
.annotate(domain=...) \
.exclude(domain='').values_list('domain', flat=True)

Im clueless about what to put in annotate().

In 1.0.x, I used a horrific hack with extra(), that has stopped
working with 1.2:

domains = UsageLogEntry.objects \
.filter(action=ACTION_FAILED_REGISTRATION,
sub_action=SUB_ACTION_DOMAIN_DISALLOWED) \
.extra(
select={ 'domain': 'SUBSTRING(username, LOCATE("@", username)+1)' },
# XXX hacks ahoy - this makes the where clause look like
# WHERE ... AND 1 HAVING `domain` != ''
where=[ "1 HAVING `domain` != ''", ],
) \
.order_by('domain') \
.values_list('domain', flat=True)

This is because (I think) in 1.0.x, the where clause was bare, and in
1.2 it is enclosed by braces, which makes my hacky insertion of a
having clause illegal syntax.

Thanks for any pointers

Tom

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