Schema::create('products', function (Blueprint $table) {
            $table->integer("id")->unique()->index();
            ...........
        });

Schema::create('purchases', function (Blueprint $table) {
            $table->id();
.........................................
            $table->integer("product_id")->unsigned()->nullable();
$table->foreign('product_id')->references('id')->on('products');

General error: 1215 Cannot add foreign key constraint (SQL: alter table `purchases` add constraint `purchases_product_id_foreign` foreign key (`product_id`) references `products` (`id`))