Bug 37419: Update FK constraint on record sources to avoid data loss

Bug 37419: Update FK constraint on record sources to avoid data loss

Without this patch, deleting a record source will delete the associated
biblio_metadata rows, which is a severe data loss.

This patch makes the constraint restrict this action.

To test:
1. Add a record source
2. Set the record source to some records
   $ koha-mysql kohadev
   > UPDATE biblio_metadata SET record_source_id='your source id' WHERE
biblionumber=1;
3. Delete the record source
=> FAIL: Record metadata deleted
4. Apply this patch
5, Run:
   $ ktd --shell
  k$ updatedatabase
=> SUCCESS: DB update goes well
6. Repeat 1~3 with another record
=> SUCCESS: Source cannot be deleted if there are linked records

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Janusz Kaczmarek <januszop@gmail.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>