[SOLVED] Django model inheritance and select

Django model inheritance and select_related

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?

Solution

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)