I have the following in models and run into strange behavior when i use select_related and model inheritance:
Models:
class A(models.Model):
field_fk = models.ForeignKey('C')
class B(A):
fields_b = models.CharField(max_length=255)
class C(models.Model):
field_c = models.CharField(max_length=255)
So A has a foreign key to C and B inherits from A.
Now I want to query A downcast it to B and read the relationship to C. To minimize sql queries I use select_related
:
obj = A.objects.select_related('b', 'field_fk).first()
obj = obj.b
print(obj.field_fk) # this prints "C object"
Because I use select_related
this should result in just one query. But somehow the information is lost during downcasting and I get to sql queries:
SELECT ••• FROM "base_a" INNER JOIN "base_c" ON
( "base_a"."field_fk_id" = "base_c"."id" ) LEFT OUTER JOIN "base_b" ON
( "base_a"."id" = "base_b"."a_ptr_id" ) ORDER BY "base_a"."id" ASC LIMIT 1
SELECT ••• FROM "base_c" WHERE "base_c"."id" = 1
So in the first query looks fine. But I am surprised that I get a second query. Is this a bug in django's ORM or am I doing something wrong?
As mentioned I submitted a ticket at django-project. https://code.djangoproject.com/ticket/25173
This is now considered as a bug and will hopefully be fixed soon.
A suggested workaround is:
obj = obj.b
print (obj.a_ptr.field_fk)