On Sat, 2006-09-23 at 18:35 +0100, Tom Smith wrote:
> I have a model that is trying to join (sloppily) across 3 tables.
> KnownCategories are a "group by catalog" reduction of Product.catalog
> (because I thought it would be quicker but I don't know if I'm
> right)...they should join like this...
>
> Product.catalog <------> KnownCategory.fk_name <------>
> MetaCategory.name
>
>
> class Product(models.Model):
> asin = models.CharField(maxlength=200)
> title = models.CharField(maxlength=200, db_index=True)
> catalog = models.CharField(maxlength=200, default='', db_index=True)
>
>
> class MetaCategory(models.Model):
> name = models.CharField(maxlength=200, core=True, db_index=True,
> unique=True)
>
> class KnownCategory(models.Model):
> meta_category = models.ManyToManyField(MetaCategory)
> fk_name = models.ForeignKey(Product, to_field='catalog')
>
>
> ...from the above... I was trying to do something like this...
>
> >>> p = Product.objects.filter(metacategory__name='Book')
Since the MetaCategory model has no direct connection to the Product
model, this query stands no chance, as you discovered. Django doesn't do
the routing through the KnownCategory model transparently (in the
general case, there isn't always going to be a unique path, for a
start). So you need to write something like
Product.objects.filter(fk_name_set__meta_category__name =
'Book')
Regards,
Malcolm
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