Make django's QuerySet.update().order_by('') work for MySQL

Summary: Since MySQL does not support a defered unique constraint (like some other databases do), it would be great if this package could supply a way to update with an order_by clause. More here. For example the

UPDATE t SET id = id + 1 ORDER BY id DESC;

example from the MySQL docs is currently not working with Django 2.2 and still gives an IntegrityError for the unique key.

The query that is executed is:

query = 'UPDATE `model` SET `integerfield` = (`model`.`integerfield` + %s) WHERE (`model`.`date` = %s AND `model`.`integerfield` >= %s)'
args = (1, '2020-04-26',  2)

As you can see the ORDER_BY, which should make it work, is omitted in the query, while it is in the Django code:

                qs_to_increase = self.get_indexing_queryset().filter(
                    integerfield__gte=self.integerfield).order_by('-integerfield')

                # apply changes after getting the qs!
                qs_to_increase.update(integerfield=F('integerfield') + 1)

Django Version: Django 2.2

Database and version used: MySQL 8.0.20-0ubuntu0.19.10.1

[adamchainz]

Thanks for the ticket, I’ve CC’d myself and added a reply.

Your instrumentation looks about right. You don’t need the table name per se, and should use += to actually mutate the local SQL variable: sql += ' ORDER BY column_name'.

[gabn88]

I just wanted to follow up on this, since upgrading to Django 3.1 breaks this way of solving it.

It is because of this commit (I believe): https://github.com/django/django/commit/a19505eb2ecd4a07ff39f3f66898e68b69d0e680#diff-dabf20e4ab58ae26d868833eefa64d93896519623d676bdb47bee47b0b6b5543 Before performing the database execution a simple “SELECT VERSION()” is issued if the connection is newly initiated. If you apply the database instrumentation hook, it will fail for not finding the table.

Fixed it by applying a

    if sql != 'SELECT VERSION()':

before I change the sql.