Add a warning about import conflicts when auto-generating migrations

[LucidComplex]

Django's automatic migration creation may suffer from a name conflict without any warning.

Creating a migration for a model having a relation with "django.contrib.auth.models.User" will cause Django to automatically import "django.conf.settings" in migrations.

This is all fine and good, unless you have an app unfortunately named "settings". Django will not fire a warning about this name conflict.

[Tim Graham]

It might be possible to automatically add some "Import x as y" shims as necessary.

[Anton Samarchyan]

So we want to have something like this if the app name is settings?

from __future__ import unicode_literals

from django.conf import settings as django_settings
from django.db import migrations, models
import django.db.models.deletion


class Migration(migrations.Migration):

    initial = True

    dependencies = [
        migrations.swappable_dependency(django_settings.AUTH_USER_MODEL),
    ]

    operations = [
        migrations.CreateModel(
            name='Test',
            fields=[
                ('id', models.AutoField(auto_created=True, primary_key=True, serialize=False, verbose_name='ID')),
                ('user', models.ForeignKey(blank=True, null=True, on_delete=django.db.models.deletion.SET_NULL, to=django_settings.AUTH_USER_MODEL)),
            ],
        ),
    ]

[Alexey Kotlyarov]

This doesn't change anything _by itself_, as an app can still be named django_settings.

[Anton Samarchyan]

What if first it's going to check if there is an app called settings, then if there is it's going to choose the name django_settings and check if the app exists again (with the name django_settings), if it does, it's going to name it django_settings_settings for example and check again, etc. Does it sound good?

[Tim Graham]

There could be more import conflicts than just one for the name "settings" so the approach should be more general. I'm not sure whether or not solving this is feasible and/or practical.

[Alexey Kotlyarov]

The needed parts of Django can be imported without from, so they stay inside django namespace:

import django.conf.settings
import django.db.migrations
import django.db.models

[Alexey Kotlyarov]

While making changes, I found that the migration tests don't check the imports inside the migration files properly, and it was possible for them to pass even though the resulting migrations wouldn't have run on their own: ​https://github.com/django/django/commit/7bd4c3e75f3d9f9bbd43c2d4a39b786f2d75137d

I have a commit that fixes the imports, but I need to figure out how to make a test for the bad application name.

[Alexey Kotlyarov]

​https://github.com/django/django/pull/7810

[Tim Graham]

I'm not sure if that's a developer friendly change considering that migrations can be authored and edited by hand. Defaulting to absolute paths everywhere makes migration files more verbose and perhaps less readable. I'd want a consensus on the DevelopersMailingList on this change before proceeding.

[Mariusz Felisiak]

I'd want a consensus on the DevelopersMailingList on this change before proceeding.

Marking as needsinfo as we should first reach a consensus about the approach.