Question

I have 2 django models:

class Foo(models.Model):
    baz = models.CharField()

class Bar(models.Model);
    foo = models.ForeignKey(Foo)
    qux = models.CharField()

with the following data:

Foo
id baz
4  X
5  Y
6  Z

Bar
id foo_id qux
1  4      A
2  5      A
3  5      B
4  6      B

now I do 2 queries on Bar, filtered on qux:

resA = [1, 2] (actually bar instances; shown bar.id for convenience)
resB = [3, 4] (actually bar instances; shown bar.id for convenience)

What is now the fastest way to AND these lists together so that the result will be:

resAND = [5] (foo.id)

Right now I do:

ret = []
nr_sets = 2
foos = Foo.objects.all()
bars = list(resA + resB)
for foo in foos:
    test = filter(lambda bar : bar.foo_id == foo.id, bars)
    if test == nr_sets;
        ret.append(foo)

This is however horribly slow. Any ideas in speeding this up? I am particularly looking for post query solutions, but good ideas concerning the query(‘s) are also welcome.

Answer 1

You could try

foo_ids = set(resA.values_list('foo_id', flat=True)) & \
    set(resB.values_list('foo_id', flat=True))

Then you can get the corresponding complete Foo objects with

ret = Foo.objects.in_bulk(foo_ids).values()

Edit: used set because apparently & does not work as expected on ValueQuerySet.