Pony.fm/database/migrations/2013_08_29_025516_create_followers.php

<?php

use Illuminate\Database\Migrations\Migration;

class CreateFollowers extends Migration {
	public function up() {
		Schema::create('followers', function($table){
			$table->increments('id');
			$table->integer('user_id')->unsigned()->index();

			$table->integer('artist_id')->unsigned()->nullable()->index();
			$table->integer('playlist_id')->unsigned()->nullable()->index();

			$table->timestamp('created_at');

			$table->foreign('user_id')->references('id')->on('users')->on_delete('cascade');
			$table->foreign('artist_id')->references('id')->on('users')->on_delete('cascade');
			$table->foreign('playlist_id')->references('id')->on('playlists');
		});
	}

	public function down() {
		Schema::drop('followers');
	}
}