Deferred foreign keys operations fail when the column is changed during the same migration

[Simon Philips]

Possibly related to #25521

The following migration is the result of squashing 3 smaller migrations together:

1. the initial migration
2. some random RunPython migration
3. changing the foreign key's column migration

The second step is only necessary to prevent squashmigrations from being smart which avoids the issue.

The squashed migration (both optimized and unoptimized) looks as following:

# -*- coding: utf-8 -*-
from __future__ import unicode_literals

from django.db import migrations, models


# bug.migrations.0002_run_python

def do_something(apps, schema_editor):
    pass  # Do nothing, it's not important anyway.


class Migration(migrations.Migration):

    replaces = [('bug', '0001_initial'), ('bug', '0002_run_python'), ('bug', '0003_change_db_column')]

    dependencies = [
    ]

    operations = [
        migrations.CreateModel(
            name='Child',
            fields=[
                ('id', models.AutoField(auto_created=True, serialize=False, verbose_name='ID', primary_key=True)),
                ('data', models.IntegerField(default=0)),
            ],
        ),
        migrations.CreateModel(
            name='Parent',
            fields=[
                ('id', models.AutoField(auto_created=True, serialize=False, verbose_name='ID', primary_key=True)),
                ('data', models.IntegerField(default=0)),
            ],
        ),
        migrations.AddField(
            model_name='child',
            name='parent',
            field=models.ForeignKey(db_column='my_parent', to='bug.Parent'),
        ),
        migrations.RunPython(
            code=do_something,
        ),
        migrations.AlterField(
            model_name='child',
            name='parent',
            field=models.ForeignKey(to='bug.Parent'),
        ),
    ]

The models are:

class Parent(models.Model):
    data = models.IntegerField(default=0)


class Child(models.Model):
    # Original db_column:
    # parent = models.ForeignKey(Parent, db_column="my_parent")

    # New db_column:
    parent = models.ForeignKey(Parent)
    data = models.IntegerField(default=0)

This will result in the following PostgreSQL error:

2015-10-08 15:00:27 CEST ERROR:  column "my_parent" does not exist
2015-10-08 15:00:27 CEST STATEMENT:  CREATE INDEX "bug_child_1e28668d" ON "bug_child" ("my_parent")

The reason seems to be that any foreign key operations are added in SQL format to the schema_editor.deferred_sql list and are then executed after all other SQL commands. However, the third operation already renamed the column for that index to 'parent_id'.

The deferred statements should follow any changes made during further operations. Or, if that's not possible, perhaps some sort of 'insert deferred statements now' operation could be added after each 'submigration' in the squashed migration?

Note that in this sample app it already crashes at the CREATE INDEX statement, but the statement after that is ALTER TABLE "bug_child" ADD CONSTRAINT "bug_child_my_parent_3b5bc08e1603165f_fk_bug_parent_id" FOREIGN KEY ("my_parent") REFERENCES "bug_parent" ("id") DEFERRABLE INITIALLY DEFERRED. I think the same issue might occur if bug_parent(id) would be renamed or removed.