On 8/3/07, Collin Grady <[EMAIL PROTECTED]> wrote:
> He wants every B that has an A fkeyed to it.
>
> In other words, every instance of B where b.a_set.count() > 0
In which case what he wants is probably something like
ModelB.objects.filter(id__in=[o.id for o in ModelA.objects.all()])
Which is kind of scary in terms of what it'll do, since it has to
instantiate every single ModelA object and do an attribute lookup on
each. Using 'values()' instead of 'all()' and specifying just the
foreign key column is a bit better, but still instantiates one
dictionary and performs one key lookup in it for each ModelA object.
Probably the most efficient thing you'll get from using just the
Django ORM methods is something like the following. Assume the models
Foo and Bar as defined here:
class Foo(models.Model):
name = models.CharField(maxlength=250)
class Bar(models.Model):
name = models.CharField(maxlength=250)
foo = models.ForeignKey(Foo)
To look up all those Foo, and only those Foo, which are currently
being referenced by a Bar, do the following:
Foo.objects.extra(where=['id IN (SELECT %s FROM %s)' % Bar._meta.db_table])
This does the whole thing in one query and avoids instantiating any
intermediate objects along the way. Note that variations in DB
implementations of SQL may mean you need to use backend.quote_name()
in there.
--
"Bureaucrat Conrad, you are technically correct -- the best kind of correct."
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