On Sep 16, 11:30 pm, Kathleen Benitez <[email protected]>
wrote:
> I am attempting to evaluate some generalization schemes using Django.
> In particular, I'd like to create a new field (generalized race), and
> then find a total population using the values() function as follows:
annotate() is somewhat confusing in your example, seeing the Record
model and explicitly stating what you want to achieve with it
would help us to help you better.
> 1 r = Records.objects
> 2 r = r.extra(select={'generalized_race: "CASE race WHEN 'Black' THEN
> 'B' WHEN 'White' THEN 'W' ELSE 'O' END"})
This is indeed "registered with Django":
>>> rs = Record.objects.extra(select={'generalized_race':
... "CASE race WHEN 'Black' THEN'B' WHEN 'White' THEN 'W' ELSE 'O'
END"})
>>> rs[0].generalized_race
u'B'
However, it is correct that although extra(select={'foo': ...})
augments the results with the field foo, it is not a proper
models.Field and can not be referenced in further QuerySet methods.
> 3 fields = ['age', 'sex', 'generalized_race']
> 4 r = r.values(*fields).annotate(Pop = Sum('population'))
Are you sure you need the values() call here? If you really don't
want to select other fields, only() works equally well and retains
the generalized_race pseudo-field:
>>> fields = ['age', 'sex']
>>> rs.only(*fields)[0].generalized_race
u'B'
As for the annotate() clause, it is unclear where the population comes
from and what's the intent. Please clarify.
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