select_related doesn't work correctly when mixing nullable and non-nullable keys
The following models:
class Model1(models.Model):
x = models.ForeignKey('Model2', null=True)
class Model2(models.Model):
y = models.ForeignKey('Model3', null=False)
class Model3(models.Model):
z = models.ForeignKey('Model4', null=False)
class Model4(models.Model):
q = models.IntegerField()
cause a problem with select_related():
In [4]: models.Model1.objects.select_related('x__y')
Out[4]: [<Model1: Model1 object>]
In [5]: models.Model1.objects.select_related('x__y__z')
Out[5]: []
It look like the fix for #7369 only fixed a case when a single non-nullable ForeignKey is encountered, not when there are two in a row.
The attached patch fixes the problem for me.