[Django] #25931: ContentTypes population doesn't work with backwards migrations

[Django]

#25931: ContentTypes population doesn't work with backwards migrations
--------------------------------------+--------------------
Reporter: amosonn | Owner: nobody
Type: Bug | Status: new
Component: contrib.contenttypes | Version: 1.9
Severity: Normal | Keywords:
Triage Stage: Unreviewed | Has patch: 0
Easy pickings: 0 | UI/UX: 0
--------------------------------------+--------------------
ContentTypes are populated after running `./manage.py migrate`. However,
the population is run as a post-migration hook, and is unaware of the
model state after the migration. Instead, it populates according to the
model state as it appears in the code, introducing stale types and
suggesting removal of live ones (from the migration's point of view).

Bug 1: When populating a new model and a GFK to it, after migrating to
before the creation of that model, the model itself will be removed, but
the CT not, and thus the GFK will be broken. When checking out the old
code (which matches the database state after the backwards migration),
enumerating and going through these GFKs will break.
In the sample project below, run the following to see breakage (make sure
to delete db.sqlite3 first):
{{{
git checkout master
./manage.py migrate
./populate1.py
./print_wrappers.py
git checkout bug1
./manage.py migrate
./print_wrappers.py
./populate2.py
./print_wrappers.py
./manage.py migrate myapp 0001 # backwards
git checkout master # now code and db should be in sync
./print_cts.py # note that CT of Second is there.
./print_wrappers.py
}}}

(sorta) Bug 2: When deleting a model, after migrating to before the
deletion of that model, the table will be created but not the contenttype.
This does not break anything, because as soon as a GFK using this CT is
created, either in the old code or during the migration, the relevant CT
is found to be missing and is created on the fly; still, this is a cover-
up, and the CT should be generated at the end of the migration.
In the sample project below, run the following to see the missing CT (make
sure to delete db.sqlite3 first):
{{{
git checkout master
./manage.py migrate
./populate1.py
./print_wrappers.py
git checkout bug2
./manage.py migrate # choose 'yes'; if you choose 'no', the next line will
break
./print_wrappers.py
./manage.py migrate myapp 0001 # backwards
git checkout master # now code and db should be in sync
./print_wrappers.py
./print_cts.py # note that CT of First is missing.
./populate1.py # this doesn't break because of the cover-up get_or_create.
./print_cts.py # now CT of First is there.
./print_wrappers.py
}}}

The sample project demonstrating both bugs is at:
https://github.com/amosonn/check_ct_populate

--
Ticket URL: <https://code.djangoproject.com/ticket/25931>
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[Django]

#25931: ContentTypes population doesn't work with backwards migrations

--------------------------------------+------------------------------------

Reporter: amosonn | Owner: nobody
Type: Bug | Status: new
Component: contrib.contenttypes | Version: 1.9

Severity: Normal | Resolution:
Keywords: | Triage Stage: Accepted
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_docs: => 0
* needs_tests: => 0
* stage: Unreviewed => Accepted

Comment:

This is related to #24100 and the tickets mentioned there, and also #24865
which suggests removing content type deletion from the `post_migrate()`
signal which should make things a bit safer. I'll accept this ticket,
though I'm not sure what exactly the remaining work will be (if any) after
those other tickets are resolved.

--
Ticket URL: <https://code.djangoproject.com/ticket/25931#comment:1>

[Django]

#25931: ContentTypes population doesn't work with backwards migrations

--------------------------------------+------------------------------------
Reporter: amosonn | Owner: nobody
Type: Bug | Status: closed
Component: contrib.contenttypes | Version: 1.9
Severity: Normal | Resolution: fixed

Keywords: | Triage Stage: Accepted
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: => fixed

Comment:

I believe this is fixed in Django 1.10 due to #24075. After `./manage.py
migrate myapp 0001 # backwards` in bug1, you'll now get a prompt to delete
stale content types. In bug 2, at `./print_cts.py # note that CT of First
is missing.` - CT of first is no longer missing.

--
Ticket URL: <https://code.djangoproject.com/ticket/25931#comment:2>