Skip AlterUniqueTogether for logger_xform

[jnm]

…and rely instead on the unique_together constraint being
automatically removed when its constituent column is dropped

Tested successfully on:

• kc.beta.kbtdev.org
• locally in a new, empty environment

[noliveleger]

It does not restore the constraint if we want to rollback.

As per discussed:

Three ideas:

• move AlterUniqueTogether after the column removal (seems like a long shot)
• add a second migration for the AlterUniqueTogether
• Create a reverse function which adds the contraint.

[jnm]

Thanks for catching this. It's not great, but it seems like it's impossible to reverse this meaningfully because:

• sms_id_string was not nullable and defaulted to an default empty string [sorry, typo];
• when we drop the column, we don't save its values anywhere;
• if we run the RemoveField backwards, it populates sms_id_string with an empty string for each logger_xform row;
• attempting to add the unique_together constraint fails for any user who has more than one form. For example, consider user 123 has two forms; the constraint would see (123, '') twice and fail with:

  django.db.utils.IntegrityError: could not create unique index "logger_xform_user_id_sms_id_string_97791222_uniq"
  DETAIL:  Key (user_id, sms_id_string)=(123, ) is duplicated.
  

I guess we could fill in unique, dummy data in sms_id_string, but would that help anyone?

[noliveleger]

I agree that it won't help. The SMS feature has been hidden/broken for a while on kobocat anyway. Thank you for the explanation.