Invalid Operation Exception MVC with Entity Framework

Question

User and Admin is a 1 to 1 relationship where there's only 1 admin role for 1 user.

Previously I fixed a ModelValidationException by adding [Key] on the first line of my administrator class following this link Click here but now I got this error:

Unable to determine the principal end of an association between the types 'RecreationalServicesTicketingSystem.Models.Administrator' and 'RecreationalServicesTicketingSystem.Models.User'. The principal end of this association must be explicitly configured using either the relationship fluent API or data annotations.

I tried following this solution but it still gave me the same error.Click Here

User.cs

public class User
    {
        public int UserID { get; set; }
        [StringLength(50, MinimumLength = 1)]
        public string LastName { get; set; }
        [StringLength(50, MinimumLength = 1, ErrorMessage = "First name cannot be longer than 50 characters.")]
        [Column("FirstName")]
        public string FirstMidName { get; set; }
        [DataType(DataType.Date)]
        [DisplayFormat(DataFormatString = "{0:yyyy-MM-dd}", ApplyFormatInEditMode = true)]
        public DateTime EnrollmentDate { get; set; }

        public int DepotID { get; set; }
        public int DepartmentID { get; set; }
        public int TicketID { get; set; }


        public string FullName
        {
            get { return LastName + ", " + FirstMidName; }
        }

        //Setting up relationships A use can apply for any number of tickets, so Tickets is defined as a collection of Ticket entities.
        public virtual ICollection<Ticket> Tickets { get; set; }

        //Setting up relationships Users can only have ONE adminstrator so we use public virtual Administrator Administrator { get; set; }
        public virtual Administrator Administrator { get; set; }
        public virtual Department Department { get; set; }

        public virtual Depot Depot { get; set; }
    }

Administrator.cs

public class Administrator
{
    [Key]
    public int AdminID { get; set; }
    public int TicketID { get; set; }   
    public int UserID { get; set; }
    [StringLength(50)]
    public string AdminRole { get; set; }
    public virtual ICollection<Ticket> Tickets { get; set; }
    public virtual User User { get; set; }
}

This is in the configuration.cs where I do the migration for user and administrator.

User

 var users = new List<User>
            {
                new User { FirstMidName = "lala",   LastName = "la", 
                    EnrollmentDate = DateTime.Parse("2016-02-18") },
                new User { FirstMidName = "baba", LastName = "baba",    
                    EnrollmentDate = DateTime.Parse("2016-02-18") },
                new User { FirstMidName = "dada",   LastName = "dada",     
                    EnrollmentDate = DateTime.Parse("2016-02-18") },
                new User { FirstMidName = "Christine",   LastName = "West",     
                    EnrollmentDate = DateTime.Parse("2016-02-18") },

            };

Administrator

    var administrator = new List<Administrator>
    {
        new Administrator {AdminID = 1, AdminRole = "Administrator LVL1", User = users.Single ( s => s.UserID == 1),
        Tickets = new List<Ticket>() },
        new Administrator {AdminID = 2, AdminRole = "Administrator LVL2", User = users.Single ( s => s.UserID == 2),
        Tickets = new List<Ticket>() },
        new Administrator {AdminID = 3, AdminRole = "Administrator LVL3", User = users.Single ( s => s.UserID == 3),
        Tickets = new List<Ticket>() }



    };
    administrator.ForEach(s => context.Administrators.AddOrUpdate(p => p.AdminID, s));
    context.SaveChanges();
}