Doc on reversible data migrations needed

[Karen Tracey]

If I create a data migration, run it, then try to migrate back to before it, I get:

  Unapplying m3app.0003_auto_20140415_1022...Traceback (most recent call last):
  File "./manage.py", line 10, in <module>
    execute_from_command_line(sys.argv)
  File "/home/kmtracey/.virtualenvs/uncfm/lib/python3.3/site-packages/django/core/management/__init__.py", line 427, in execute_from_command_line
    utility.execute()
  File "/home/kmtracey/.virtualenvs/uncfm/lib/python3.3/site-packages/django/core/management/__init__.py", line 419, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "/home/kmtracey/.virtualenvs/uncfm/lib/python3.3/site-packages/django/core/management/base.py", line 288, in run_from_argv
    self.execute(*args, **options.__dict__)
  File "/home/kmtracey/.virtualenvs/uncfm/lib/python3.3/site-packages/django/core/management/base.py", line 337, in execute
    output = self.handle(*args, **options)
  File "/home/kmtracey/.virtualenvs/uncfm/lib/python3.3/site-packages/django/core/management/commands/migrate.py", line 145, in handle
    executor.migrate(targets, plan, fake=options.get("fake", False))
  File "/home/kmtracey/.virtualenvs/uncfm/lib/python3.3/site-packages/django/db/migrations/executor.py", line 62, in migrate
    self.unapply_migration(migration, fake=fake)
  File "/home/kmtracey/.virtualenvs/uncfm/lib/python3.3/site-packages/django/db/migrations/executor.py", line 114, in unapply_migration
    migration.unapply(project_state, schema_editor)
  File "/home/kmtracey/.virtualenvs/uncfm/lib/python3.3/site-packages/django/db/migrations/migration.py", line 121, in unapply
    raise Migration.IrreversibleError("Operation %s in %s is not reversible" % (operation, self))
django.db.migrations.migration.IrreversibleError: Operation <RunPython <function load_initial_data at 0x7f7af2424050>> in m3app.0003_auto_20140415_1022 is not reversible

I cannot find any documentation on how to create a reversible data migration. I'm used to doing this with South, I assume there is some way to do this in the new migrations framework? We need some doc on how to do it...or if this is not just a missing doc issue then this needs to be re-classified as a migrations code issue...

[Karen Tracey]

Peeking at the code reveals RunPython can be handed a 2nd callable to do the "reverse" operation. So I think this doc:

​https://docs.djangoproject.com/en/1.7/topics/migrations/#data-migrations

specifically needs to be updated to include mention of that, at a minimum.

[Tim Graham]

It's covered in the ​RunPython reference, but I will add a couple sentences to the topic guide.

[Tim Graham <timograham@…>]

In 1ce759b69f641fd780790630c79559fc68a2d923:

Fixed #22445 -- Added how to perform a reverse data migration to topic guide.

Thanks Karen Tracey for the report.

[Tim Graham <timograham@…>]

In 23f3c53aff54b7f298f29e97042d00dbd9379d00:

[1.7.x] Fixed #22445 -- Added how to perform a reverse data migration to topic guide.

Thanks Karen Tracey for the report.

Backport of 1ce759b69f from master