app\Product::categories PHP Method

    public function categories()
    {
        return $this->belongsTo('App\\Category', 'category_id');
    }

 /**
  * Store a newly created resource in storage.
  *
  * @param  \Illuminate\Http\Request  $request
  * @return \Illuminate\Http\Response
  */
 public function store(Request $request)
 {
     $this->validate($request, ['product_title' => 'required']);
     $product = new Product();
     $product->is_published = $request->get('is_published') == 'on' ? 1 : 0;
     $product->title = $request->get('product_title');
     $product->description = $request->get('product_description');
     $product->min_player = $request->get('product_input-players-min');
     $product->max_player = $request->get('product_input-players-max');
     $product->min_age = $request->get('product_input-age-min');
     $product->cost_price = $request->get('product_costprice');
     $product->sale_price = $request->get('product_saleprice');
     $product->save();
     if ($request->get('product_categories') != null) {
         foreach ($request->get('product_categories') as $category_id) {
             $product->categories()->attach($category_id);
         }
     }
     $files = $request->file('product_images');
     $count = 0;
     foreach ($files as $file) {
         if ($file == null) {
             continue;
         }
         $product->addMedia($file)->usingFileName($product->id . "_" . $count . "." . $file->getClientOriginalExtension())->toCollection('images');
         $count++;
     }
     Session::flash('flash_message', 'Product successfully added!');
     return view('dashboard/create_product')->with('categories', Category::get())->with('product', new Product());
 }