Add a model for likes

migrations/2018-05-10-154336_create_likes/down.sql

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+-- This file should undo anything in `up.sql`
+DROP TABLE likes;

migrations/2018-05-10-154336_create_likes/up.sql

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+-- Your SQL goes here
+CREATE TABLE likes (
+    id SERIAL PRIMARY KEY,
+    user_id INTEGER REFERENCES users(id) ON DELETE CASCADE NOT NULL,
+    post_id INTEGER REFERENCES posts(id) ON DELETE CASCADE NOT NULL,
+    creation_date TIMESTAMP NOT NULL DEFAULT now()
+)