Fixed #12240 - select_related doesn't work correctly when mixing nullable and non-nullable keys

            avoid = avoid_set.copy()

            dupe_set = orig_dupe_set.copy()

            table = f.rel.to._meta.db_table

            if nullable or f.null:

                promote = True

            else:

                promote = False

            promote = nullable or f.null

            if model:

                int_opts = opts

                alias = root_alias

                next = requested.get(f.name, {})

            else:

                next = False

            if f.null is not None:

                new_nullable = f.null

            else:

                new_nullable = None

            new_nullable = f.null or promote

            for dupe_opts, dupe_col in dupe_set:

                self.query.update_dupe_avoidance(dupe_opts, dupe_col, alias)

            self.fill_related_selections(f.rel.to._meta, alias, cur_depth + 1,

from django.db import models

class SystemDetails(models.Model):

    details = models.TextField()

class SystemInfo(models.Model):

    system_details = models.ForeignKey(SystemDetails)

    system_name = models.CharField(max_length=32)

class Forum(models.Model):

__test__ = {'API_TESTS':"""

>>> s = SystemInfo.objects.create(system_name='First forum')

>>> d = SystemDetails.objects.create(details='First details')

>>> s = SystemInfo.objects.create(system_name='First forum', system_details=d)

>>> f = Forum.objects.create(system_info=s, forum_name='First forum')

>>> p = Post.objects.create(forum=f, title='First Post')

>>> c1 = Comment.objects.create(post=p, comment_text='My first comment')

>>> Comment.objects.select_related('post').filter(post__isnull=True)[0].post is None

True

>>> comments = Comment.objects.select_related('post__forum__system_info__system_details')

>>> [(c.id, c.comment_text, c.post) for c in comments]

[(1, u'My first comment', <Post: First Post>), (2, u'My second comment', None)]

"""}