Django: get ValueQuerySet with values from generic relation

I have two models linked by a generic relation:

from django.contrib.contenttypes import generic
from django.db import models

class Bar(models.Model):
    content_type   = models.ForeignKey(ContentType)
    object_id      = models.PositiveIntegerField()
    content_object = generic.GenericForeignKey()

    code  = models.CharField(max_length=10)
    value = models.IntegerField()

class Foo(models.Model):
    ... base fields ...
    bars = generic.GenericRelation(Bar)

and now I want to get all the 'bar.value's whose 'code is 'xxx', with a syntax like:

Foo.objects.filter(...foofilters..., bars__code='xxx').values('bars__value')

but this doesn't work (Django tells me that 'bars__value' is not a valid field).

Any hint?

EDIT: in SQL I would have done something like this:

SELECT bar.value
FROM   foo 
JOIN   django_content_type AS ct
    ON ct.app_label = 'foo_app'
   AND ct.model = 'foo'
JOIN   bar 
    ON bar.content_type_id = ct.id
   AND bar.object_id = foo.id
WHERE  bar.code = 'xxx'

or taking content_type_id with another query

You cannot do it like that... Imaging how sql should look like for that case

What you might need is:

foo_ids = Foo.objects.filter(foo-filters).values_list('id', flat=True) # Getting Foo ids filtered
Bar.objects.filter(
          content_type=ContentType.objects.get_for_model(Foo),
          object_id__in=foo_ids,
          bar-filters
    ).values('value')