public class Department {

        [Key]
        public int ID { get; set; } = 0;

        [Required]
        public string Name { get; set; } = "";

        public virtual ICollection<User> Users { get; set; }
    }

public class User {

        [Key]
        public int ID { get; set; } = 0;

        public int DepartID { get; set; } = 0;

        [StringLength(30)]
        public string Username { get; set; } = "";

        [ForeignKey("DepartID")]
        public virtual Department Department { get; set; }
    }

protected override void Seed(GCSecurity.MVC.DAL.GCDbContext context) {

            var departs = new List<Department> {
                new Department{ ID = 1, Name = "办公室" }
            };
            articles.ForEach(s => context.Articles.Add(s));
            context.SaveChanges();

            context.Users.Add(new User {
                ID = 1,
                Username = "admin",
                DepartID = 1
            });
            context.SaveChanges();
}