[Django] #29290: Previously squashed migration fails under Python 3 with "multiple leaf nodes" error
Django
nodes" error
--------------------------------------+------------------------
Reporter: Ed Morley | Owner: nobody
Type: Bug | Status: new
Component: Migrations | Version: 1.11
Severity: Normal | Keywords:
Triage Stage: Unreviewed | Has patch: 0
Needs documentation: 0 | Needs tests: 0
Patch needs improvement: 0 | Easy pickings: 0
UI/UX: 0 |
--------------------------------------+------------------------
== Context ==
Whilst Django 2.0 / master have dropped support for Python 2, for many
existing Python 2.7 projects the common path to the ideal "Django 2 +
Python 3" end state will be an interim step of "Django 1.11 + Python 3".
As such, it seems in the best interests of making that transition as easy
as possible, to reduce the number of people left behind on older Django
versions.
== STR ==
1. Clone [https://github.com/edmorley/testcase-django-squashmigrations-
py3compat testcase-django-squashmigrations-py3compat]
2. `mkvirtualenv dj111-py2 -p python2.7`
3. `pip install Django==1.11.12`
4. `./manage.py squashmigrations testapp 0002 --noinput`
5. `./manage.py migrate`
6. `mkvirtualenv dj111-py3 -p python3.6`
7. `pip install Django==1.11.12`
8. `./manage.py migrate`
== Expected ==
The migrate command at step 8 completes successfully, or else a less
misleading error message shown.
== Actual ==
Step 8 results in:
{{{#!python
CommandError: Conflicting migrations detected; multiple leaf nodes in the
migration graph:
(0002_some_change, 0001_squashed_0002_some_change in testapp).
To fix them run 'python manage.py makemigrations --merge'
}}}
Running the suggested `manage.py makemigrations --merge` fails too (and it
shouldn't be necessary anyway):
{{{#!python
Traceback (most recent call last):
...
File ".../django/core/management/commands/makemigrations.py", line 272,
in handle_merge
raise ValueError("Could not find common ancestor of %s" %
migration_names)
ValueError: Could not find common ancestor of
{'0001_squashed_0002_some_change', '0002_some_change'}
}}}
== Additional notes ==
* If the bytestring prefix (`b'...'`) is removed from the migration names
specified in `replaces` in the generated squashed migration, the migration
succeeds.
* If prior to squashing the migration, `from __future__ import
unicode_literals` is added to `settings.py`, then the generated migration
does not contain bytestring prefixes. Adding to `settings.py` is necessary
since this project uses the default `AppConfig` rather than a manually
specified one.
* The above was using Python 2.7.14 and Python 3.6.4.
* Related tickets #25906 and #24949, though they seem broader in scope,
and don't reference the misleading "multiple leaf nodes" error
specifically (which was one of the most confusing aspects when debugging
this).
== Thoughts on fixes ==
* Ideally the `migrate` command's graph traversal would coerce all
migration names to string before comparing, which would make the above
"just work" even on previously created squashed migrations.
* Alternatively the `squashmigration` command could coerce the migration
names specified in `replaces` to string before even under Python 2, to
ensure the generated migration is correct. However this will still mean
existing projects have to make manual edits to existing migrations.
* It might also be worth adding `from __future__ import unicode_literals`
to the `startproject` template `settings.py` to catch cases where people
don't point at a custom `AppConfig`.
* Failing all of the above, it would be good to make the error message
less misleading and/or mention this case explicitly in the docs.
Especially since the current docs suggested workaround of adding
`unicode_literals` and re-running `makemigrations` doesn't work for
squashed migrations (it still results in the `multiple leaf nodes` error
unless the squashed migration is hand-edited). Docs in question:
* https://docs.djangoproject.com/en/1.11/topics/migrations/#supporting-
python-2-and-3
* https://docs.djangoproject.com/en/1.11/topics/python3/
Side note: It's not clear whether an app not having a specified
`AppConfig` is fine, or considered legacy/deprecated. If the latter it
would be great to clarify this in the docs, and state the disadvantages or
even add a deprecation warning.
--
Ticket URL: <https://code.djangoproject.com/ticket/29290>
Django <https://code.djangoproject.com/>
The Web framework for perfectionists with deadlines.
Django
nodes" error
Reporter: Ed Morley | Owner: nobody
Type: Bug | Status: new
Component: Migrations | Version: 1.11
Keywords: | Triage Stage: Unreviewed
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
Comment (by Tim Graham):
As far as I see, your proposals may not have much value for versions other
than Django 1.11 which is mostly receiving only security and data loss
fixes at this time. At least there's now this ticket to document the
problem for anyone who searches for it.
--
Ticket URL: <https://code.djangoproject.com/ticket/29290#comment:1>
Django
nodes" error
Reporter: Ed Morley | Owner: nobody
Type: Bug | Status: new
Component: Migrations | Version: 1.11
Keywords: | Triage Stage: Unreviewed
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
Comment (by Ed Morley):
Agreed.
Perhaps the best fix would be to update
[https://docs.djangoproject.com/en/1.11/topics/migrations/#supporting-
python-2-and-3 this page] to recommend manually removing the bytestring
attributes in existing migrations (after adding `from __future__ import
unicode_literals` everywhere) rather than running `makemigrations`?
--
Ticket URL: <https://code.djangoproject.com/ticket/29290#comment:2>
Django
nodes" error
Reporter: Ed Morley | Owner: nobody
Type: Bug | Status: new
Component: Migrations | Version: 1.11
Keywords: | Triage Stage: Unreviewed
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
Comment (by Tim Graham):
Generally, we don't update (except for critical fixes) the non-stable
versions of the documentation to avoid introducing untranslated text into
the translated documentation. I'm not sure if this is worth an exception.
--
Ticket URL: <https://code.djangoproject.com/ticket/29290#comment:3>
Django
nodes" error
Reporter: Ed Morley | Owner: nobody
Component: Migrations | Version: 1.11
Severity: Normal | Resolution: wontfix
Keywords: | Triage Stage: Unreviewed
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
Changes (by Tim Graham):
* status: new => closed
* resolution: => wontfix
Comment:
I guess I'd be willing to review a documentation patch if you want to
provide it.
--
Ticket URL: <https://code.djangoproject.com/ticket/29290#comment:4>