Hi Tom,
Whilst Django annotation is great, it is rather simplistic in nature
and can only do very basic Count, Sum, Avg, etc. which do not allow
any filtering (i.e. a WHERE clause in the SQL).
Unless there are major differences in 1.2 (which I haven't seen) then
you won't be able to do what you want I'm afraid.
Euan
On May 28, 10:26 am, Tom Evans <tevans...@googlemail.com> wrote:
> 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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