Create null xform instances for DailyXFormSubmissionCounter

[LMNTL]

Description

DailyXFormSubmissionCounter doesn't actually have a pre_delete signal, so the xform=NULL counter isn't getting updated when xforms are deleted. This made sense when it got deleted after 31 days, but now it's our primary way of figuring out submission data on the usage page.

This PR resolves that by making the DailyXFormSubmissionCounter function more like the MonthlyXFormSubmissionCounter:

• Adding a user field
• Adding unique constraints between user, date, and xform (or just the first two when xform=NULL)
• Adding a pre_delete signal to update the null counter

A separate PR for kpi will handle updating the shadow model (ReadOnlyDailyXFormSubmissionCounter).

Notes

I tried to update any place where daily counters are created to include the user. Because this includes the populate_submission_counters management command, I reordered migrations so that the schema changes happen before that command is run.

[noliveleger]

Let's use single quotes ;-)

[noliveleger]

' vs "

[noliveleger]

I think this subquery is wrong. It updates all the rows with the same user (actually that's the case when I test it).

Looking closer at the SQL query makes it more obvious.

UPDATE "logger_dailyxformsubmissioncounter"
   SET "user_id" = (
        SELECT U1."user_id"
          FROM "logger_dailyxformsubmissioncounter" U0
         INNER JOIN "logger_xform" U1
            ON (U0."xform_id" = U1."id")
         WHERE NOT (U0."xform_id" IS NULL)
         LIMIT 1
       )
 WHERE NOT ("logger_dailyxformsubmissioncounter"."xform_id" IS NULL)

Correct me if I'm wrong, but the subquery always returns the xform owner of the first daily counter.

I think you need to use a OuterRef over here.

DailyXFormSubmissionCounter.objects.all().exclude(xform=None).update(
    user=Subquery(
        DailyXFormSubmissionCounter.objects.filter(pk=OuterRef('pk')).values('xform__user')[:1]
    ),
)

With the OuterRef, the where conditions of the subquery seems more accurate.

UPDATE "logger_dailyxformsubmissioncounter"
   SET "user_id" = (
        SELECT U1."user_id"
          FROM "logger_dailyxformsubmissioncounter" U0
          LEFT OUTER JOIN "logger_xform" U1
            ON (U0."xform_id" = U1."id")
         WHERE U0."id" = "logger_dailyxformsubmissioncounter"."id"
         LIMIT 1
       )
 WHERE NOT ("logger_dailyxformsubmissioncounter"."xform_id" IS NULL)