DbUpdateExceptionC# + EntityFramework

Имею класс билета

        public int Id { get; set; }
        public byte Semester { get; set; }
        public int CollegeId { get; set; }
        [ForeignKey("CollegeId")]
        public virtual College College { get; set; }
        public int AssociateDirectorId { get; set; }
        [ForeignKey("AssociateDirectorId")]
        public  virtual AssociateDirector AssociateDirector { get; set; }
        public int SpecialtyId { get; set; }
        [ForeignKey("SpecialtyId")]
        public virtual  Specialty Specialty { get; set; }
        public int CyclicalCommissionId { get; set; }
        [ForeignKey("CyclicalCommissionId")]
        public virtual CyclicalCommission CyclicalCommission { get; set; }
        public virtual ICollection<Subject> Subjects { get; set; }
        public virtual ICollection<Teacher> Teachers { get; set; }
        public virtual ICollection<Question> Questions { get; set; }

Я генерирую такие билеты, выбирая экземпляры teachers,subjects и рандомно вытягивая questions из бд, но при попытке сохранения получаю Exception. College и Teacher относятся друг к другу как многие к одному. Т.е у Teacher есть CollegeId а у College есть список объектов Teacher.

System.Data.Entity.Infrastructure.DbUpdateException: "Unable to determine the principal end of the 'Examens.DataBase.Teacher_College' relationship. Multiple added entities may have the same primary key.

Код заполнения такой

                for(int i = 0; i< count;i++)
                {
                    /*Логика заполнения*/
                    Card card = new Card()
                    {
                        AssociateDirector = (Directors.SelectedItem as AssociateDirector),
                        College = (Colleges.SelectedItem as College),
                        CyclicalCommission = (Comissions.SelectedItem as CyclicalCommission),
                        Specialty = (Specialties.SelectedItem as Specialty),
                        Questions = cardQuestions,
                        Semester = byte.Parse(Semester.Text),
                        Subjects =  subjects,
                        Teachers =  teachers
                    };
                    cards.Add(card);
                }
                using (ExamsDbContext edc = new ExamsDbContext())
                {
                    edc.Cards.AddRange(cards);
                    edc.SaveChanges();
                }

А вот код классов, которые я вообще использую

    public class AssociateDirector
    {
        public int Id { get; set; }
        [Required] [MaxLength(30)] public string Name { get; set; }
        [Required] [MaxLength(30)] public string Surname { get; set; }
        [Required] [MaxLength(30)] public string LastName { get; set; }
        public int CollegeID { get; set; }
        public virtual College College { get; set; }
        public virtual ICollection<Card> Cards { get; set; }
        public override string ToString()
        {
            return string.Format("{0} {1}.{2}.", Surname, Name[0], LastName[0]);
        }
    }


    public class Chairman
    { 
        public int Id { get; set; }
        [Required] [MaxLength(30)] public string Name { get; set; }
        [Required] [MaxLength(30)] public string Surname { get; set; }
        [Required] [MaxLength(30)] public string LastName { get; set; }
        public virtual ICollection<CyclicalCommission> CyclicalCommissions { get; set; }
        public override string ToString()
        {
            return string.Format("{0} {1}.{2}.", Surname, Name[0], LastName[0]);
        }
    }


    public class College
    {
        public int Id { get; set; }
        [Required] [MaxLength(150)]public string Name { get; set; }
        public virtual  ICollection<AssociateDirector> AssociateDirectors { get; set; }
        public virtual ICollection<CyclicalCommission> cyclicalCommissions { get; set; }
        public virtual  ICollection<Specialty> Specialties { get; set; }
        public virtual  ICollection<Teacher> Teachers { get; set; }
        public virtual ICollection<Card> Cards { get; set; }
        public override string ToString()
        {
            return Name;
        }
    }


    public class CyclicalCommission
    {
        //TODO: CyclicalCommission is ready
        public int Id { get; set; }
        [Required] [MaxLength(100)] public string Name { get; set; }
        public int ChairmanId { get; set; }
        public int CollegeId { get; set; }
        [ForeignKey("CollegeId")]
        public virtual College College { get; set; }
        public virtual Chairman Chairman { get; set; }
        public virtual ICollection<Card> Cards { get; set; }

        public override string ToString()
        {
            return Name;
        }
    }


    public class Question
    {
        //TODO: Question is ready
        public int Id { get; set; }
        [Required] public string Text { get; set; }
        public byte[] Image { get; set; }
        public int Complexity { get; set; }
        public int SubjectId { get; set; }
        [ForeignKey("SubjectId")]
        public virtual Subject Subject { get; set; }
        public virtual ICollection<Card> Cards { get; set; }
    }


    public class Specialty
    {
        public int Id { get; set; }
        [Required] [StringLength(8)] public string Code { get; set; }

        public virtual ICollection<College> Colleges { get; set; }
        public virtual ICollection<Subject> Subjects { get; set; }
        public virtual ICollection<Card> Cards { get; set; }
        public override string ToString()
        {
            return Code;
        }
    }


    public class Subject
    {
        public int Id { get; set; }
        [Required] [MaxLength(100)] public string Name { get; set; }
        public virtual ICollection<Teacher> Teachers { get; set; }
        public virtual  ICollection<Specialty> Specialties { get; set; }
        public virtual ICollection<Question> Questions { get; set; }
        public virtual  ICollection<Card> Cards { get; set; }
        public override string ToString()
        {
            return Name;
        }
    }


    public class Teacher
    {
        public int Id { get; set; }
        [Required] [MaxLength(30)] public string Name { get; set; }
        [Required] [MaxLength(30)] public string Surname { get; set; }
        [Required] [MaxLength(30)] public string LastName { get; set; }
        public int CollegeId { get; set; }
        [ForeignKey("CollegeId")]
        [DatabaseGenerated(DatabaseGeneratedOption.None)]
        public virtual College College { get; set; }
        public virtual ICollection<Subject> Subjects { get; set; }  
        public virtual  ICollection<Card> Cards { get; set; }
        public override string ToString()
        {
            return string.Format("{0} {1}.{2}.", Surname, Name[0], LastName[0]);
        }
    }