On 8 Oct, 01:31, Russell Keith-Magee <freakboy3...@gmail.com> wrote:
> On Thu, Oct 8, 2009 at 1:43 AM, Emily Rodgers
>
>
>
> <emily.kate.rodg...@googlemail.com> wrote:
>
> > Hello,
>
> > I am a bit stuck on something that I think ought to be really easy (so
> > I am probably being really stupid!).
>
> > I am trying to write a view for a model such that I can pass the view
> > (via post data) a (or an unknown) number of fields of that model, and
> > receive back a list of dictionaries of distinct values for those
> > fields and the number of times that combination of values appears.
>
> > So, suppose the model was for holding data about cars, and it had
> > fields 'manufacturer', 'model', 'fuel_type', 'colour', 'n_of_doors',
> > etc where the fields had appropriate field types (a mixture of
> > different field types), I would want to be able to pass the view for
> > example groupBy=['colour', 'fuel_type'], and it would return a dict of
> > distinct colour/fuel type combinations and how many cars there are in
> > the db with those combinations, eg. [{'colour': 'red', 'fuel_type:
> > 'diesel', 'count': 14}, ...].
>
> > To me it seems like the kind of thing you would want to use annotate
> > and Count for, except Count takes one field not multiple fields.
>
> Depending on the exact result you're looking for, this may not be a
> problem. The following query may do the job:
>
> Car.objects.values('make','model').annotate(count=Count('id'))
>
> will give you a list of (make, model, count) indicating how many
> instances of each make-model pair there are.
>
> The complication here is exactly what you want to count. Are you looking for:
> * The number of rows that have every make/model combination?
> or
> * The number of distinct combinations for every make/model combination?
>
> Consider the following data:
>
> Ford | Explorer | Black
> Ford | Explorer | Black
> Ford | Explorer | Blue
> Ford | Explorer | Red
> Dodge | Charger | Red
>
> The query I gave you will return (Ford, Explorer, 4), (Dodge, Charger,
> 1). However, if you want to collapse the two "black Ford Explorer"
> entries and only return a count of 3, you're out of luck. For that you
> need to specify multiple columns to the Count(), which Django doesn't
> support. If this is what you need, you'll need to fall back on using
> raw SQL.
>
> Yours,
> Russ Magee %-)
Ah your suggestion is exactly what I am after :)
Although, I think if there are foreign keys (or choices for the field)
etc, values() tends to return the id of the value, which means I have
to go figure out which model it relates to, then find that record, and
get the user friendly data (I am returning this using JSON to a ExtJS
script). This starts to make it expensive again.
I will have a play with your suggestion though - thanks :)
Emily
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