ALTER TABLE pull_requests DROP CONSTRAINT pull_requests_pkey;
ALTER TABLE pull_requests DROP CONSTRAINT pull_requests_pr_id_key;

ALTER TABLE pull_requests ADD CONSTRAINT pull_requests_pkey PRIMARY KEY (pr_id, issue_id);
ALTER TABLE pull_requests ADD CONSTRAINT pull_requests_pr_id_key UNIQUE (pr_id, issue_id);
ALTER TABLE pull_requests ADD CONSTRAINT pull_requests_fkey FOREIGN KEY (issue_id) REFERENCES issues(issue_id);