Improve unique constraint for slugs by considering deleted entries
As reported by @dchatzic , users can currently not recreate entries after deleting them due to 500 Internal Server Error
.
This is due to the fact that the unique key constraint only considers the slug
column, but we keep deleted entries in the same database.
This patch extends the unique key constraint to cover both slug
and deletion_timestamp
columns (and also adds an appropriate integration test).