<?php

use Illuminate\Database\Migrations\Migration;
use Illuminate\Database\Schema\Blueprint;
use Illuminate\Support\Facades\Schema;

class CreateWatches extends Migration
{
    /**
     * Run the migrations.
     *
     * @return void
     */
    public function up()
    {
        Schema::create('watches', function (Blueprint $table) {
            $table->bigIncrements('id');
            $table->bigInteger('user_id')->unsigned();
            $table->string('notifiable_type');
            $table->bigInteger('notifiable_id')->unsigned();
            $table->string('subtype');
            $table->timestampsTz();

            $table->unique(['user_id', 'notifiable_type', 'notifiable_id', 'subtype']);
            $table->index(['notifiable_type', 'notifiable_id']);
        });
    }

    /**
     * Reverse the migrations.
     *
     * @return void
     */
    public function down()
    {
        Schema::drop('watches');
    }
}

<?php

use Illuminate\Database\Migrations\Migration;
use Illuminate\Database\Schema\Blueprint;
use Illuminate\Support\Facades\Schema;

class CreateWatches extends Migration
{
    /**
     * Run the migrations.
     *
     * @return void
     */
    public function up()
    {
        Schema::create('watches', function (Blueprint $table) {
            $table->bigIncrements('id');
            $table->bigInteger('user_id')->unsigned();
            $table->enum('notifiable_type', ['beatmapset', 'build', 'news_post']);
            $table->bigInteger('notifiable_id')->unsigned();
            $table->string('subtype');
            $table->timestampsTz();

            $table->unique(['user_id', 'notifiable_type', 'notifiable_id', 'subtype']);
            $table->index(['notifiable_type', 'notifiable_id']);
        });
    }

    /**
     * Reverse the migrations.
     *
     * @return void
     */
    public function down()
    {
        Schema::drop('watches');
    }
}

Use enum for the type
Use enum for the type
