Pony.fm/database/migrations/2013_08_01_051337_create_comments.php

<?php

use Illuminate\Database\Migrations\Migration;

class CreateComments extends Migration
{
    public function up()
    {
        Schema::create('comments', function ($table) {
            $table->increments('id');
            $table->integer('user_id')->unsigned();
            $table->string('ip_address', 46);
            $table->text('content');

            $table->timestamps();
            $table->timestamp('deleted_at')->nullable()->index();

            $table->integer('profile_id')->unsigned()->nullable()->index();
            $table->integer('track_id')->unsigned()->nullable()->index();
            $table->integer('album_id')->unsigned()->nullable()->index();
            $table->integer('playlist_id')->unsigned()->nullable()->index();

            $table->foreign('profile_id')->references('id')->on('users');
            $table->foreign('user_id')->references('id')->on('users');
            $table->foreign('track_id')->references('id')->on('tracks');
            $table->foreign('album_id')->references('id')->on('albums');
            $table->foreign('playlist_id')->references('id')->on('playlists');
        });
    }

    public function down()
    {
        Schema::table('comments', function ($table) {
            $table->dropForeign('comments_user_id_foreign');
            $table->dropForeign('comments_track_id_foreign');
            $table->dropForeign('comments_album_id_foreign');
            $table->dropForeign('comments_playlist_id_foreign');
        });
        Schema::drop('comments');
    }
}