On Wed, Feb 18, 2009 at 11:12 AM, Deniz Dogan <[email protected]> wrote:
> Let's say I have a model called Bike with a DateField called
> "production_date". Now I want to get all of the Bikes and group them
> by their production date. How would I do this in Django? I can't seem
> to figure it out.
For this type of query, you need to ask yourself two things:
The first question is easy: What kind of aggregate do I want? You want
a count, so you'd use the ``Count()`` aggregate (i.e.
``django.db.models.Count``). That's simple; I'll bet you already
figured that part out.
The second part is more tricky: what is the set of fields I'm grouping
over? By default, it's all the fields on the model, so a simple
``aggregate(Count(production_date))`` basically is the same as a
``COUNT(*)``, which you don't want::
>>> Bike.objects.aggregate(Count('production_date'))
{'production_date__count': 7}
So you need to change the list of fields you're grouping over. You do
this by using ``values()``; the important part of the docs to read is
http://docs.djangoproject.com/en/dev/topics/db/aggregation/#values;
make sure to pay attention to the part about the order of
``annotate()`` and ``values()``.
If you read that carefully, you'll see that to change the set of
grouped fields you'll want to call ``values()`` before calling
``annotate()``::
>>> qs = Bike.objects.values('production_date') \
... .annotate(count=Count('production_date')) \
>>> for r in qs:
... print "%(production_date)s: %(count)s bikes" % r
2008-01-01 2 bikes
2008-01-02 1 bikes
2008-01-03 1 bikes
2008-01-04 2 bikes
2008-01-05 1 bikes
Hope that helps!
Jacob
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