#23226: Migrations generated by python2 break when using python3 to migrate
---------------------------------+----------------------------------------
Reporter: morshed.nader@… | Owner: andrewgodwin
Type: Bug | Status: assigned
Component: Migrations | Version: 1.7-rc-2
Severity: Release blocker | Resolution:
Keywords: | Triage Stage: Accepted
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
---------------------------------+----------------------------------------
Comment (by charettes):
Replying to [comment:6 andrewgodwin]:
> I'm in two minds as to whether we should just force-convert everything
to Unicode in migration files (which obviously is technically changing
data, and might break with strings intended for humans to read), or
document that you should be using unicode for your models files.
>
> There's a potential third outcome here where we fix the state stuff to
convert all model names/field names to unicode, which would solve most
immediate problems. I'll investigate.
The thing is that it's hard to which part needs fixing to solve the
immediate problem. Take `order_with_respect_to` for example:
{{{#!python
# models.py
from django.db import models
class A(models.Model): pass
class B(models.Model):
a = models.ForeignKey(A)
class Meta:
order_with_respect_to = 'a'
}}}
{{{#!python
# migrations/0001_initial.py (generated on Py2)
from __future__ import unicode_literals
from django.db import models, migrations
class Migration(migrations.Migration):
dependencies = [
]
operations = [
migrations.CreateModel(
name='A',
fields=[
('id', models.AutoField(verbose_name='ID',
serialize=False, auto_created=True, primary_key=True)),
],
options={
},
bases=(models.Model,),
),
migrations.CreateModel(
name='B',
fields=[
('id', models.AutoField(verbose_name='ID',
serialize=False, auto_created=True, primary_key=True)),
('a', models.ForeignKey(to='app.A')),
],
options={
},
bases=(models.Model,),
),
migrations.AlterOrderWithRespectTo(
name='b',
order_with_respect_to=b'a',
),
]
}}}
It would also fail with a similar exception when migration on Python 3
{{{#!python
Operations to perform:
Apply all migrations: auth, app, sessions, contenttypes, migratemodule,
admin
Running migrations:
Applying app.0001_initial...Traceback (most recent call last):
File "./manage.py", line 11, in <module>
execute_from_command_line(sys.argv)
File "/home/simon/workspace/django/django/core/management/__init__.py",
line 330, in execute_from_command_line
utility.execute()
File "/home/simon/workspace/django/django/core/management/__init__.py",
line 322, in execute
self.fetch_command(subcommand).run_from_argv(self.argv)
File "/home/simon/workspace/django/django/core/management/base.py", line
363, in run_from_argv
self.execute(*args, **cmd_options)
File "/home/simon/workspace/django/django/core/management/base.py", line
413, in execute
output = self.handle(*args, **options)
File
"/home/simon/workspace/django/django/core/management/commands/migrate.py",
line 160, in handle
executor.migrate(targets, plan, fake=options.get("fake", False))
File "/home/simon/workspace/django/django/db/migrations/executor.py",
line 63, in migrate
self.apply_migration(migration, fake=fake)
File "/home/simon/workspace/django/django/db/migrations/executor.py",
line 91, in apply_migration
if self.detect_soft_applied(migration):
File "/home/simon/workspace/django/django/db/migrations/executor.py",
line 135, in detect_soft_applied
apps = project_state.render()
File "/home/simon/workspace/django/django/db/migrations/state.py", line
67, in render
model.render(self.apps)
File "/home/simon/workspace/django/django/db/migrations/state.py", line
292, in render
body,
File "/home/simon/workspace/django/django/db/models/base.py", line 287,
in __new__
new_class._prepare()
File "/home/simon/workspace/django/django/db/models/base.py", line 312,
in _prepare
opts._prepare(cls)
File "/home/simon/workspace/django/django/db/models/options.py", line
170, in _prepare
self.order_with_respect_to =
self.get_field(self.order_with_respect_to)
File "/home/simon/workspace/django/django/db/models/options.py", line
388, in get_field
raise FieldDoesNotExist('%s has no field named %r' %
(self.object_name, name))
django.db.models.fields.FieldDoesNotExist: B has no field named b'a'
}}}
I think that all field and model options referring to field names somehow
should be converted to `six.text_type`. Idem with `dict` based options
(such as `limit_choices_to`) that might be passed as `**kwargs` or string
based ones that might end up as a kwarg name since `callable(**{b'key':
'value'})` isn't allowed on Python 3 (While both `bytes` and `unicode`
kwarg is allowed on Python 2).
Since those criteria covers most of the existing field and model options I
suggest we convert all of them (during deconstruction) instead of trying
to guess every possible places where this option might be referenced.
This alternative might also help mitigate issues users on Python 2
creating migrations and then later trying to move to Python 3 would
encounter.
--
Ticket URL: <https://code.djangoproject.com/ticket/23226#comment:7>
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