#34178: Prefetching a foreign key on GenericForeignKey results in incorrect
queryset being selected
-------------------------------------+-------------------------------------
Reporter: Rohan | Owner: nobody
Nahata |
Type: Bug | Status: new
Component: Database | Version: 4.1
layer (models, ORM) | Keywords: GenericForeignKey,
Severity: Normal | prefetch_related
Triage Stage: | Has patch: 0
Unreviewed |
Needs documentation: 0 | Needs tests: 0
Patch needs improvement: 0 | Easy pickings: 0
UI/UX: 0 |
-------------------------------------+-------------------------------------
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.
--
Ticket URL: <https://code.djangoproject.com/ticket/34178>
Django <https://code.djangoproject.com/>
The Web framework for perfectionists with deadlines.
--
You received this message because you are subscribed to the Google Groups
"Django updates" group.
To unsubscribe from this group and stop receiving emails from it, send an email
to [email protected].
To view this discussion on the web visit
https://groups.google.com/d/msgid/django-updates/010701849f27132a-6a2fd1f7-b8c6-4b6b-afce-30c1cdd5c0da-000000%40eu-central-1.amazonses.com.
