useful of impload function

when you need to use map function while imploading 
	function implodeProduct($product){
		$multiplied = $product->stocks->map(function ($item, $key) {
			return $item->size .' = '.$item->qty ;
		});
		return $multiplied->implode(', <br>');
	}
and model 
    // each product might have many product 
    public function stocks()
    {
        return $this->hasMany(Stock::class,'product_id');
    }