Update migrations.py

[antoonhuiskens]

Added "IF EXISTS" to the DROP CONSTRAINT statement intending to drop the {LTSS.tablename}_pkey.

I've opened issue #111 for tracking this.

DROP CONSTRAINT IF EXISTS capability exists in PostgreSQL versions going back to at least version 11 (oldest supported version by postgreSQL)

Tested this against postgreSQL 15.

[Coffee0297]

Can confirm this works with the missing table when dropping. tested with latest version of LTSS

[freol35241]

Thank you @antoonhuiskens . This looks reasonable, however it would be good to try to understand why this is necessary. Do you have any idea?

This is also related to the (OT?) discussion in #100

[antoonhuiskens]

I suspect I had a few passes at installing due to backing out the upgrade and down revving to the previous version in order to get things working again. Looking at the code, it seems reasonable to assume that caused the initial deletion of the key.

Since this is a distinct issue unrelated to compression as per #100, I figured it would be helpful to create a separate issue and fix.
/A

[antoonhuiskens]

A more general approach would likely be to wrap the entire set of upgrade changes to the db in a a BEGIN/COMMIT block, with a ROLLBACK in case of errors, so a robust way would be to autocommit=False for all sql, plus some code to handle the commit. Out of scope for this PR though.

That should (in theory) prevent the db from getting into an undefined state.