Re: [Django] #24860: Migrations leak stale RelatedManager related_model definitions

[Django]

#24860: Migrations leak stale RelatedManager related_model definitions
-+-
 Reporter:  ecederstrand |Owner:  nobody
 Type:  Bug  |   Status:  closed
Component:  Migrations   |  Version:  1.8
 Severity:  Normal   |   Resolution:  needsinfo
 Keywords:  migration,   | Triage Stage:
  RelatedManager |  Unreviewed
Has patch:  0|  Needs documentation:  0
  Needs tests:  0|  Patch needs improvement:  0
Easy pickings:  0|UI/UX:  0
-+-
Changes (by timgraham):

 * status:  new => closed
 * resolution:   => needsinfo


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The Web framework for perfectionists with deadlines.

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Re: [Django] #24860: Migrations leak stale RelatedManager related_model definitions

[Django]

#24860: Migrations leak stale RelatedManager related_model definitions
-+-
 Reporter:  ecederstrand |Owner:  nobody
 Type:  Bug  |   Status:  new
Component:  Migrations   |  Version:  1.8
 Severity:  Normal   |   Resolution:
 Keywords:  migration,   | Triage Stage:
  RelatedManager |  Unreviewed
Has patch:  0|  Needs documentation:  0
  Needs tests:  0|  Patch needs improvement:  0
Easy pickings:  0|UI/UX:  0
-+-
Changes (by timgraham):

 * Attachment "24860-test.diff" added.


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Ticket URL: 
Django 
The Web framework for perfectionists with deadlines.

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Re: [Django] #24860: Migrations leak stale RelatedManager related_model definitions

[Django]

#24860: Migrations leak stale RelatedManager related_model definitions
-+-
 Reporter:  ecederstrand |Owner:  nobody
 Type:  Bug  |   Status:  new
Component:  Migrations   |  Version:  1.8
 Severity:  Normal   |   Resolution:
 Keywords:  migration,   | Triage Stage:
  RelatedManager |  Unreviewed
Has patch:  0|  Needs documentation:  0
  Needs tests:  0|  Patch needs improvement:  0
Easy pickings:  0|UI/UX:  0
-+-
Changes (by timgraham):

 * needs_better_patch:   => 0
 * needs_tests:   => 0
 * needs_docs:   => 0


Comment:

 I tried to reproduce with a test based on the steps you outlined, but
 didn't have any luck (also tried similar steps in a sample project). Could
 you try tweaking the test until it fails or submit a sample project that
 reproduces the crash?

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[Django] #24860: Migrations leak stale RelatedManager related_model definitions

[Django]

#24860: Migrations leak stale RelatedManager related_model definitions
--+---
 Reporter:  ecederstrand  |  Owner:  nobody
 Type:  Bug   | Status:  new
Component:  Migrations|Version:  1.8
 Severity:  Normal|   Keywords:  migration, RelatedManager
 Triage Stage:  Unreviewed|  Has patch:  0
Easy pickings:  0 |  UI/UX:  0
--+---
 I have a Django 1.8.2 app with models A and B, where data in class A has
 been migrated from one field to another. My actual code is a lot more
 involved and difficult to reduce to a sane test case, but the following is
 the essence of my debugging:

 {{{
 # Unchanged
 class B(models.Model):
pass


 # version 1
 class A(models.Model):
b = models.ForeignKey(B)
x = models.FooField()
 }}}

 Now field 'y' is added to class A and a migration is created

 {{{
 # version 2
 class A(models.Model):
rel = models.ForeignKey(B)
x = models.FooField()
y = models.BarField()
 }}}

 A data migration to move data from 'x' to 'y' is created

 {{{
A = apps.get_model("my_app", "A")
for i in A.objects.all():
   i.y = i.x
   i.save()
 }}}

 Field 'x' is no longer needed on class A. New migration is created

 {{{
 # version 3
 class A(models.Model):
rel = models.ForeignKey(B)
y = models.BarField()
 }}}

 Now when I run a test case which uses prefetch, invalid SQL is generated.

 {{{
 def test_case():
 list(B.objects.all().prefetch_related('a_set'))
 ->  psycopg2.ProgrammingError: column A.x does not exist
 }}}


 The reason for this is that 'def related_manager_cls()' in
 django/db/models/fields/related.py:832 is a @cached_property. When I run
 my test case, the migrations are run to create the test database. This
 caches version 2 of class A in self.related.related_model as a , which is then returned later on in the test case. Thus,
 prefetch SQL for version 2 is produced instead of for version 3. I suspect
 the problem occurs because of the data migration.

 No errors show when I use prefetch_related() in production, because the
 migrations aren't run.

 I'm uncertain how to fix this. Somehow the property caches should be
 invalidated after each migration is run.

 It seems related.py has more potentally dangerous uses of @cached_property

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