multi-table inheritance: accessing child object after select_related('child') causes extra query

[Sebastian Illing]

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 two 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?

I also posted this at stackoverflow, but I am quite sure that this is a bug.
​http://stackoverflow.com/questions/31628588/django-model-inheritance-and-select-related

[Shai Berger]

Could you please verify the following:

• After

  obj = A.objects.select_related('b', 'field_fk).first()
  

  You can traverse the FK without incurring another database query;
• You should be able to save the query by explicitly using the "a" object:

     obj = obj.b        
     print (obj.a_ptr.field_fk)
  

but I'd agree that, even if it works, that's a workaround for something that's worth solving.

[Sebastian Illing]

obj = A.objects.select_related('b', 'field_fk).first()

• yes there is only one query

   obj = obj.b        
   print (obj.a_ptr.field_fk)

• this also results in one query.

But unfortunately I can't do that in my actual use case.

[Baptiste Mispelon]

As Shai said, it's probably worth looking into fixing this.

Attached is a reproduction testcase for this issue.

[Baptiste Mispelon]

Reproduction testcase

[Tim Graham]

Looks like a duplicate of #16043.

[V.P.]

This ticket is not a duplicate of #16043, and should be reopened.
Ticket #16043 is about accessing parent model from child, this ticket is about the opposite operation.
This problem still exists in Django 3.2, and probably in later versions too.

[Mariusz Felisiak]

Please provide a reproducible scenario before reopening a ticket. Tests provided by Baptist pass on the current main branch (and 3.2).

[V.P.]

Mariusz Felisiak, sorry, it looks like I had this behavior because of another issue in code. Now I cannot reproduce it, everything works fine.