Change a constraint to add ON DELETE CASCADE

I've not used these in many places, to try and avoid hiding deleting data, but
in this case, this will allow more easily deleting the derivation source file
nars, by just deleting the derivation_source_files table entry.

sqitch/deploy/change_derivation_source_file_nars_constraint.sql

@@ -0,0 +1,13 @@
+-- Deploy guix-data-service:change_derivation_source_file_nars_constraint to pg
+
+BEGIN;
+
+ALTER TABLE derivation_source_file_nars
+  DROP CONSTRAINT derivation_source_file_nars_derivation_source_file_id_fkey;
+
+ALTER TABLE derivation_source_file_nars
+  ADD CONSTRAINT derivation_source_file_nars_derivation_source_file_id_fkey
+  FOREIGN KEY (derivation_source_file_id) REFERENCES derivation_source_files(id)
+  ON DELETE CASCADE;
+
+COMMIT;

sqitch/revert/change_derivation_source_file_nars_constraint.sql

@@ -0,0 +1,7 @@
+-- Revert guix-data-service:change_derivation_source_file_nars_constraint from pg
+
+BEGIN;
+
+-- XXX Add DDLs here.
+
+COMMIT;

sqitch/sqitch.plan

@@ -70,3 +70,4 @@ add-tsvectors-per-locale 2020-07-16T18:19:44Z daniela <daniela@linux-ijv5> # Add
 change_package_descriptions_index 2020-08-23T11:56:00Z Christopher Baines <mail@cbaines.net> # Change the package_descriptions index
 change_locale_values 2020-09-26T10:20:22Z Christopher Baines <mail@cbaines.net> # Change locale values
 change_autovacuum_config 2020-10-01T21:24:46Z Christopher Baines <mail@cbaines.net> # Change autovacuum config
+change_derivation_source_file_nars_constraint 2020-10-02T17:12:58Z Christopher Baines <mail@cbaines.net> # Change derivation source file nars constraint

sqitch/verify/change_derivation_source_file_nars_constraint.sql

@@ -0,0 +1,7 @@
+-- Verify guix-data-service:change_derivation_source_file_nars_constraint on pg
+
+BEGIN;
+
+-- XXX Add verifications here.
+
+ROLLBACK;