Hm, I thought it was kind of self-evident. Okay, here's the sample
Model definition:
class Records (models.Model):
id = models.IntegerField(primary_key=True)
state = models.TextField()
race = models.TextField()
age = models.IntegerField()
sex = models.TextField()
population = models.IntegerField()
Imagine it has 5 records
1 WA Black 55 F 123
2 WA White 55 F 556
3 WA Asian 55 F 612
4 WI Amer. Indian 55 F 125
5 WA Unknown 55 F 23
So I know the population for a combination of demographics (from
census data, fwiw, though the example above is totally fabricated).
But I want to combine some groups and see what the new population
distribution is. In this case, I want to create a generalized race
grouping, and I want to end up with something like this
Black 55 F 123
White 55 F 556
Other 55 F 760
My current approach is to add an extra field with the generalized race
classification
r = r.extra(select={'generalized_race: "CASE race WHEN 'Black' THEN
'B' WHEN 'White' THEN 'W' ELSE 'O' END"})
As mrts said, this does create a new field, but the field is
unavailable to use in further queries. Below is what I would *like*
to work....Add the new field, then use the values() function to group
by all applicable fields.
fields = ['age', 'sex', 'generalized_race']
r = r.values(*fields).annotate(Pop = Sum('population'))
Any suggestions for alternate approaches?
~Kathy
On Sep 17, 3:29 am, mrts <mrts.py...@gmail.com> wrote:
> On Sep 16, 11:30 pm, Kathleen Benitez <kathleen.beni...@gmail.com>
> 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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