How to create Auto generated column in Entity framework 6

Hi,

I want to create Auto generated column in Entity framework 6 using Code first approach

 public class Shopping
    {
        [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
        [Key]

        public int ProductId { get; set; }
        public string ProductCategory { get; set; }
        public string ProductName { get; set; }
        public string ProductQuantity { get; set; }
        public string Price { get; set; }

        //public virtual Items Item { get; set; }
    }

this is not creating auto generated key on ProductId column.