-- Delete orphaned replies so the foreign key we're about to add doesn't freak
-- out:
CREATE TEMPORARY TABLE tweets_to_delete (id int)
    SELECT child.id FROM customercare_tweet child
        LEFT JOIN customercare_tweet parent ON child.reply_to=parent.tweet_id
        WHERE parent.id IS NULL AND
              child.reply_to IS NOT NULL;
DELETE FROM customercare_tweet WHERE id IN (SELECT id FROM tweets_to_delete);

-- Remove surrogate key, and use tweet ID instead. Make reply_to a foreign key,
-- and rename it to conform to our conventions.
ALTER TABLE customercare_tweet DROP id, ADD PRIMARY KEY (tweet_id);
ALTER TABLE customercare_tweet CHANGE reply_to reply_to_id bigint;
ALTER TABLE customercare_tweet ADD CONSTRAINT reply_to_id_refs_tweet_id_47b7f44d FOREIGN KEY (reply_to_id) REFERENCES customercare_tweet (tweet_id);

-- Drop old tweet_id index, now redundant with pkey:
ALTER TABLE customercare_tweet DROP KEY tweet_id;