Add creation timestamps

migrations/2018-04-30-170445_timestamps/down.sql

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+-- This file should undo anything in `up.sql`
+ALTER TABLE posts DROP COLUMN creation_date;
+ALTER TABLE blogs DROP COLUMN creation_date;
+ALTER TABLE users DROP COLUMN creation_date;
+ALTER TABLE instances DROP COLUMN creation_date;

migrations/2018-04-30-170445_timestamps/up.sql

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+-- Your SQL goes here
+ALTER TABLE posts ADD COLUMN creation_date TIMESTAMP NOT NULL DEFAULT now();
+ALTER TABLE blogs ADD COLUMN creation_date TIMESTAMP NOT NULL DEFAULT now();
+ALTER TABLE users ADD COLUMN creation_date TIMESTAMP NOT NULL DEFAULT now();
+ALTER TABLE instances ADD COLUMN creation_date TIMESTAMP NOT NULL DEFAULT now();