How to write migrations?
We only write up
migrations, down
migrations are not always possible in sqlite or
too complex/too expensive. For example to remove a column you would have to duplicate
the table, copy over the data, delete and recreated.
This can be very expensive for some tables (user_messages for example), so should not be attempted.
Notes
One issue we faced multiple times is that updates to user_messages
can be very
expensive, leading to slow upgrade times and interrupted migrations. So avoid
writes if not necessary.