Django AutoField wrong migration for "id" field - django

Question

I have quite simple Django model:
class MyModel(models.Model):
user = models.ForeignKey(User)
somestring = models.CharField(
max_length=250
)
... some other string fields...
There is no declared "id" field in model, so it has automatic primary key assigned by Django.
Initial migration looked like this:
migrations.CreateModel(
name='MyModel',
fields=[
('user', models.ForeignKey(to=settings.AUTH_USER_MODEL)),
('somestring', models.CharField(max_length=250))
... some other string fields...
],
options={},
bases=(models.Model,),
),
It was successfully applied and database table contains fields like:
- id <== autogenerated
- user
- somestring
etc...
Occasionally I've started to get messages like
Your models have changes that are not yet reflected in a migration, and so won't be applied.
Run 'manage.py makemigrations' to make new migrations, and then re-run 'manage.py migrate' to apply them.
After running manage.py makemigrations it generated very strange migration:
$ ./manage.py makemigrations
You are trying to add a non-nullable field 'id' to mymodel without a default; we can't do that (the database needs something to populate existing rows).
Please select a fix:
1) Provide a one-off default now (will be set on all existing rows)
2) Quit, and let me add a default in models.py
Select an option: 1
Please enter the default value now, as valid Python
The datetime and django.utils.timezone modules are available, so you can do e.g. timezone.now()
>>>
Please enter some code, or 'exit' (with no quotes) to exit.
>>> ''
Migrations for 'myapp':
0036_auto_20150623_1535.py:
- Add field id to mymodel
It loosk like this:
class Migration(migrations.Migration):
dependencies = [
('foo', 'bar'),
]
operations = [
migrations.AddField(
model_name='mymodel',
name='id', <== !!!
field=models.AutoField(auto_created=True, primary_key=True, default='', serialize=False, verbose_name='ID'),
preserve_default=False,
),
]
But this migration doesn't makes sense and sure it fails, because there is already "id" field in appropriate DB table.
The quick and dirty solution is to --fake this migration. Which should work locally on dev machine, but can result into migration errors on other environments (test/staging/prod).
Looks like the model's old/new fields state was calculated incorrectly, so that "id" wasn't included in old model but it is included in new one, so django descided that id field should be added.
Not sure about the cause, but my main question is - What is the proper approach to reset the model or migration state and return it to the previous one, when there is no any need in such migration?

Answer 1

Alright, found a viable workaround for this.
Added "id" field declaration into original migration, so it is no longer reported as missing.
migrations.CreateModel(
name='MyModel',
fields=[
!!===> ('id', models.AutoField(verbose_name='ID', serialize=False, auto_created=True, primary_key=True)),
('user', models.ForeignKey(to=settings.AUTH_USER_MODEL)),
('somestring', models.CharField(max_length=250))
... some other string fields...
],
options={},
bases=(models.Model,),
),
This change will not be applied on environments where we already have this migration and new environments should get the proper db table structure from the start.
TIL: Django DOES NOT use SQL to fetch the current model state from DB, - but create mock of the model and applies the migrations to that mock until it gets the latest version of model according all available migration operations. And then that reconstructed model is compared to real one from models.py.
Adjusting the original migration solved the issue. Thanks to everyone who participated in discussions.