How to fix "Cannot insert explicit value for identity column" error?

Question

SQL

There are two entities

public class Sub
    {
        public int SubId { get; set; }
        public string Name { get; set; }
        public SubType Type { get; set; }
        public int Priority { get; set; }
        public int SupGroupId { get; set; }
        public virtual SubGroup SubGroup { get; set; }
        public enum SubType { TextBox,TextArea,Image};
    }

public class SubGroup
    {
        public int SubGroupId { get; set; }
        public string Name { get; set; }
        public Type ForType { get; set; }
        public int Priority { get; set; }
        public virtual ICollection<Sub> Sub { get; set; }

        public enum Type
        {
            [Display(Name = "Отзывы")]
            Review,
            [Display(Name = "Предложения")]
            Application,
            [Display(Name = "Страницы")]
            Page,
            [Display(Name = "Организации")]
            Organization,
            [Display(Name = "Тендеры")]
            Tender,
            [Display(Name = "Тендеры")]
            Offer,
            [Display(Name = "Комментарии")]
            Comment,
            [Display(Name = "Посты")]
            Post
        }
    }

Adding SubGroup to DbSet happens without error. But when setting the SubGroup property on the Sub model, the following Exception is thrown

Answer 1

on each auto-incrementing (by means of subd) field, you must add the [DatabaseGenerated (DatabaseGeneratedOption.Identity)] attribute
for one for all primary key fields, put the [Key] attribute