Pivot Table Laravel

public function up() {
    Schema::create('book_category', function (Blueprint $table) {
        $table->increments('id');
        $table->integer('book_id')->unsigned()->nullable();
        $table->integer('category_id')->unsigned()->nullable();
        $table->timestamps(); $table->foreign('book_id')->references('id')->on('books');
        $table->foreign('category_id')->references('id')- >on('categories');
    });
}