On Oct 8, 8:36 am, Emily Rodgers <emily.kate.rodg...@googlemail.com>
wrote:
> 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
Realised I was being dumb, and you can follow the foreign keys to get
more useful values by specifying them when you pass the strings to
values() :-)
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