Looks like we've got to delete or null out Message.parent for orphans.

lpmain_staging=# -- Delete the Message rows.
lpmain_staging=# DELETE FROM Message USING MessagesToDelete
lpmain_staging-#  WHERE Message.id = MessagesToDelete.message;
ERROR:  update or delete on table "message" violates foreign key constraint "message_parent_fk" on table "message"
DETAIL:  Key (id)=(6664529) is still referenced from table "message".
lpmain_staging=# 
lpmain_staging=# -- Delete the bad BugWatch rows.
lpmain_staging=# DELETE FROM BugWatch USING BugsWithBadBugWatches
lpmain_staging-#  WHERE BugsWithBadBugWatches.bug = BugWatch.bug
lpmain_staging-#    AND BugWatch.bugtracker = (
lpmain_staging(#     SELECT id FROM BugTracker WHERE name = 'debbugs')
lpmain_staging-#    AND BugWatch.owner = (
lpmain_staging(#     SELECT id FROM Person WHERE name = 'bug-watch-updater')
lpmain_staging-#    AND BugWatch.datecreated > (
lpmain_staging(#     TIMESTAMP WITH TIME ZONE '2010-03-31 1100Z')
lpmain_staging-#    AND BugWatch.datecreated < (
lpmain_staging(#     TIMESTAMP WITH TIME ZONE '2010-03-31 2300Z');
ERROR:  current transaction is aborted, commands ignored until end of transaction block
lpmain_staging=# 
lpmain_staging=# -- Delete the bad BugCve rows. This could overreach because there is
lpmain_staging=# -- no ownership information in BugCve.
lpmain_staging=# DELETE FROM BugCve USING BugsWithBadBugWatches
lpmain_staging-#  WHERE BugsWithBadBugWatches.bug = BugCve.bug
lpmain_staging-#    AND BugCve.date_created > (
lpmain_staging(#     TIMESTAMP WITH TIME ZONE '2010-03-31 1100Z')
lpmain_staging-#    AND BugCve.date_created < (
lpmain_staging(#     TIMESTAMP WITH TIME ZONE '2010-03-31 2300Z');
ERROR:  current transaction is aborted, commands ignored until end of transaction block
lpmain_staging=# 
lpmain_staging=# -- Delete the bad BugActivity rows.
lpmain_staging=# DELETE FROM BugActivity USING BugsWithBadBugWatches
lpmain_staging-#  WHERE BugsWithBadBugWatches.bug = BugActivity.bug
lpmain_staging-#    AND BugActivity.person = (
lpmain_staging(#     SELECT id FROM Person WHERE name = 'bug-watch-updater')
lpmain_staging-#    AND BugActivity.datechanged > (
lpmain_staging(#     TIMESTAMP WITH TIME ZONE '2010-03-31 1100Z')
lpmain_staging-#    AND BugActivity.datechanged < (
lpmain_staging(#     TIMESTAMP WITH TIME ZONE '2010-03-31 2300Z')
lpmain_staging-#    AND BugActivity.whatchanged IN ('bug watch added', 'cve linked');
ERROR:  current transaction is aborted, commands ignored until end of transaction block