Opened 4 years ago

Closed 4 years ago

Last modified 4 years ago

#19073 closed Bug (worksforme)

strange behaviour of select_related

Reported by: anonymous Owned by: nobody
Component: Database layer (models, ORM) Version: 1.4
Severity: Normal Keywords:
Cc: Triage Stage: Unreviewed
Has patch: no Needs documentation: no
Needs tests: no Patch needs improvement: no
Easy pickings: no UI/UX: no


Sometimes Django uses inner join instead of left outer join in queries with select_related. This breaks queries with nullable OneToOne fields.
Here's the example


class A(models.Model):

class B(models.Model):

class C(models.Model):
    b = models.OneToOneField(B)
    a = models.ForeignKey(A)

class D(models.Model):
    b = models.OneToOneField(B, null=True)


[<D: D object>, <D: D object>, <D: D object>] #ok
>>> D.objects.select_related('b').all()
[<D: D object>, <D: D object>, <D: D object>] #ok
>>> D.objects.select_related('b', 'b__c').all()
[<D: D object>, <D: D object>, <D: D object>] #ok
>>> D.objects.select_related('b', 'b__c__a').all()
[] #WRONG! empty result
>>> print D.objects.select_related('b', 'b__c__a').all().query
SELECT "app1_d"."id", "app1_d"."b_id", "app1_b"."id", "app1_c"."id", "app1_c"."b_id", "app1_c"."a_id", "app1_a"."id" FROM "app1_d" LEFT OUTER JOIN "app1_b" ON ("app1_d"."b_id" = "app1_b"."id") LEFT OUTER JOIN "app1_c" ON ("app1_b"."id" = "app1_c"."b_id") INNER JOIN "app1_a" ON ("app1_c"."a_id" = "app1_a"."id")

INNER JOIN "app1_a" should be LEFT OUTER JOIN "app1_a" imo

Change History (4)

comment:1 Changed 4 years ago by Aymeric Augustin

Needs documentation: unset
Needs tests: unset
Patch needs improvement: unset
Resolution: worksforme
Status: newclosed

Here's the result I get on master with the models described above:

>>> from test19073.models import *
>>> D.objects.create()
<D: D object>
>>> D.objects.all()
[<D: D object>]
>>> D.objects.select_related('b').all()
[<D: D object>]
>>> D.objects.select_related('b', 'b__c').all()
[<D: D object>]
>>> D.objects.select_related('b', 'b__c__a').all()
[<D: D object>]
>>> D.objects.select_related('b', 'b__c__b').all()
[<D: D object>]

This bug may have been fixed after 1.4, or your report may be incomplete.

Please reopen if you can reproduce the bug on master.

comment:2 Changed 4 years ago by anonymous

Resolution: worksforme
Status: closednew
Summary: strange behaviuor of select_relatedstrange behaviour of select_related

With the model defined above, the following test fails (django 1.4):

def test_bug19073_ABCD(self):
    nb_of_objects = 10
    {D.objects.create() for _i in xrange(nb_of_objects)}
    self.assertEqual(len(D.objects.all()), nb_of_objects) # ok
    self.assertEqual(len(D.objects.select_related('b').all()), nb_of_objects) # ok
    self.assertEqual(len(D.objects.select_related('b__c').all()), nb_of_objects) # ok
    self.assertEqual(len(D.objects.select_related('b__c__a').all()), nb_of_objects) # nok
    self.assertEqual(len(D.objects.select_related('b__c__b').all()), nb_of_objects) # nok

A similar problem is with the following model (OneToOneField 'hidden' in the inheritance):

class R(models.Model):

class P(models.Model):

class Q(P):
    r = models.ForeignKey(R)
class O(models.Model):
    p = models.ForeignKey(P, null=True)

and with the following test:

def test_bug19073_OPQR(self):
    nb_of_objects = 10
    {O.objects.create() for _i in xrange(nb_of_objects)}
    self.assertEqual(len(O.objects.all()), nb_of_objects) # ok
    self.assertEqual(len(O.objects.select_related('p').all()), nb_of_objects) # ok
    self.assertEqual(len(O.objects.select_related('p__q').all()), nb_of_objects) # ok
    self.assertEqual(len(O.objects.select_related('p__q__r').all()), nb_of_objects) # nok

In every "nok" cases, there is a INNER JOIN in the underlying query that messes things up.

Strangely enough, note that the following "simpler" model J->K->L->M (no inheritance as opposed to O->P=>Q->R and without going reverse C->B as in D->B, C->{B,A} above), works:

class M(models.Model): #rush

class L(models.Model): #captured_snapshot
    m = models.ForeignKey(M)

class K(models.Model): #snapshot
    l = models.OneToOneField(L)
class J(models.Model): #talk
    k = models.OneToOneField(K, null=True)

Tests are all ok here:

def test_bug19073_JKLM(self):
    nb_of_objects = 10
    {J.objects.create() for _i in xrange(nb_of_objects)}
    self.assertEqual(len(J.objects.all()), nb_of_objects) # ok
    self.assertEqual(len(J.objects.select_related('k').all()), nb_of_objects) # ok
    self.assertEqual(len(J.objects.select_related('k__l').all()), nb_of_objects) # ok
    self.assertEqual(len(J.objects.select_related('k__l__m').all()), nb_of_objects) # ok <- strange!

comment:3 Changed 4 years ago by anonymous

Could this be a regression of #7369? It seems related...

comment:4 Changed 4 years ago by Anssi Kääriäinen

Resolution: worksforme
Status: newclosed

We are not going to fix this in 1.4. Our backporting policy is such that only critical bug fixes are applied to released versions.

aaugustin tried to reproduce this on master and the queries worked. There have been some improvements in select_related for 1.5. So, it is likely that this has been fixed in 1.5.

Re-closing as worksforme.

EDIT: As a workaround you can add a filter like: Q(bcaisnull=True)|Q(bcaisnull=False) to the query. The filter doesn't actually filter anything out but it forces a LEFT join for the problematic join.

Version 1, edited 4 years ago by Anssi Kääriäinen (previous) (next) (diff)
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