id	summary	reporter	owner	description	type	status	component	version	severity	resolution	keywords	cc	stage	has_patch	needs_docs	needs_tests	needs_better_patch	easy	ui_ux
34178	Prefetching a foreign key on GenericForeignKey results in incorrect queryset being selected	Rohan Nahata	nobody	"These are the models that I created to test my hypothesis. . 
{{{
class Foreign(models.Model):
    value = models.CharField(max_length=20)

class OtherForeign(models.Model):
    value = models.CharField(max_length=20)

class A(models.Model):
    f = models.ForeignKey(Foreign, on_delete=models.PROTECT)

class B(models.Model):
    f = models.ForeignKey(OtherForeign, on_delete=models.PROTECT)

class X(models.Model):
    ct = models.ForeignKey(ContentType, on_delete=models.PROTECT)
    object_id = models.IntegerField()
    object = GenericForeignKey(ct_field='ct', fk_field='object_id')
}}}

The tables were populated with this sample data. 
{{{
Foreign

id|value
1|A1
2|A2
3|A3
4|A4
5|A5
6|A6
7|A7
8|A8
9|A9
10|A10


Other Foreign

id|value
1|B1
2|B2
3|B3
4|B4
5|B5
6|B6
7|B7
8|B8
9|B9
10|B10

A

id|f_id
1|1
2|2
3|3
4|4
5|5
6|6
7|7
8|8
9|9
10|10

B

id|f_id
1|1
2|2
3|3
4|4
5|5
6|6
7|7
8|8
9|9
10|10

X
id|object_id|ct_id
1|1|11
2|2|11
3|3|11
4|4|11
5|5|11
6|6|11
7|7|11
8|8|11
9|9|11
10|10|11
11|1|10
12|2|10
13|3|10
14|4|10
15|5|10
16|6|10
17|7|10
18|8|10
19|9|10
20|10|10

}}}

Based on this data : 

{{{
->> xx = X.objects.prefetch_related('object').all().order_by('id')
->> xx[0].object.f.value
'A1'
->> xx[10].object.f.value
'B1'
}}} 
This behaviour is correct. 

However, when we chain another prefetch to the same,
{{{
->> xx = X.objects.prefetch_related('object', 'object__f').all().order_by('id')
->> xx[0].object.f.value
'A1'
->> xx[10].object.f.value
'A1'
}}}
Here, {{{xx[10].object}}} returns the correct object, however, when {{{xx[10].object.f}}} is evaluated it refers to model A instead of model B. This leads me to believe that there is some sort of faulty caching going on in the background that leads to model A being used. 

This bug was tested on 4.1.3, 3.2.16, 2.2.28 and was reproducible. "	Bug	closed	Database layer (models, ORM)	4.1	Normal	duplicate	GenericForeignKey, prefetch_related	rohanahata@…	Unreviewed	0	0	0	0	0	0