Many-to-many Entity framework and repository pattern Insert/Update

Question

I am using EF code first approach with fluent api. I am having one registration form in my application where in registering candidate can select multiple options from dropdown(Interested In Dropdown on Sign-Up form) that has a predefined set of options (which may increase in future but the chances are very rare). When the user submits the form I want to save this records to database. So I created following entities.

Participant Class where the registering candidates information will be saved

public class Participant
    {
        public Participant()
        {
            Interests = new Collection<Interest>();
        }
        [Key, ForeignKey("User")]
        public int Id { get; set; }
        [DisplayName("First Name")]
        [StringLength(50, ErrorMessage = "First name cannot be more than 50 characters")]
        [Required(ErrorMessage = "You must fill in first name")]
        public string FirstName { get; set; }

        [DisplayName("Last Name")]
        [StringLength(50, ErrorMessage = "Last name cannot be more than 50 characters")]
        [Required(ErrorMessage = "You must fill in last name")]
        public string LastName { get; set; }

        [Required(ErrorMessage = "You must indicate your full birthday")]
        [DisplayName("Birthday")]
        [DataType(DataType.DateTime)]
        public DateTime BirthDate { get; set; }

        [DisplayName("Gender")]
        [Required(ErrorMessage = "You must select gender")]
        public int Gender { get; set; }

        public string Address { get; set; }

        public int CountryId { get; set; }
        public Country Country { get; set; }

        [DisplayName("Zip code")]
        [StringLength(10, ErrorMessage = "Zip code cannot be more than 10 characters")]
        public string ZipCode { get; set; }

        public string Mobile { get; set; }

        public string PhotoUrl { get; set; }

        public virtual User User { get; set; }

        public virtual ICollection<Interest> Interests { get; set; }

        public string MedicalConditions { get; set; }
}

Interest Class from where the Interested In dropdown on Sign-up form will get populate The user can select multiple options from the Interested In dropdown

Interest Class

 public class Interest
    {
        public Interest()
        {
            Participants = new Collection<Participant>();
        }
        public int Id { get; set; }
        public string InterestName { get; set; }
        public virtual ICollection<Participant> Participants { get; private set; }
    }

To hold each participants interest I created a ParticipantInterests table in DB with following schema. ParticipantInterests Id (PK) ParticipantId (FK from Participants table) InterestId (FK Interests table)

I added public virtual ICollection Participants { get; set; } in Interest model and

public virtual ICollection Interests { get; set; } in Participant model to form Many-To-Many association. My Data Context class is as follows

public class STNDataContext : DbContext
    {
        public DbSet<Participant> Participants { get; set; }
        public DbSet<User> Users { get; set; }
        public DbSet<Country> Countries { get; set; }
        public DbSet<Interest> Interests { get; set; }
        public DbSet<Role> Roles { get; set; }
        public DbSet<SecurityQuestion> SecurityQuestions { get; set; }

        public DbSet<Tour> Tours { get; set; }
        protected override void OnModelCreating(DbModelBuilder modelBuilder)
        {
            modelBuilder.Entity<Participant>()
                .HasKey(p => p.Id);

            modelBuilder.Entity<User>()
                .HasOptional(u => u.Participant)
                .WithRequired();

            modelBuilder.Entity<Participant>()
            .HasMany(p => p.Interests)
            .WithMany(i => i.Participants)
            .Map(m =>
            {
                m.ToTable("ParticipantInterests");
                m.MapLeftKey("ParticipantId");
                m.MapRightKey("InterestId");
            });

            modelBuilder.Entity<User>().HasRequired(u => u.Role);
            modelBuilder.Entity<Participant>().HasRequired(p => p.Country);
            modelBuilder.Entity<User>().HasOptional(u => u.SecurityQuestion);
        }


        public virtual void Commit()
        {
            base.SaveChanges();
        }

Controller Action Code

public virtual ActionResult Register(StudentRegisterViewModel studentRegisterViewModel)
        {
            if (ModelState.IsValid)
            {
                if (_userService.IsUserExists(studentRegisterViewModel.Participant.User) == false)
                {
                    studentRegisterViewModel.Participant.User.Username = studentRegisterViewModel.Username;
                    studentRegisterViewModel.Participant.User.Email = studentRegisterViewModel.Email;
                    studentRegisterViewModel.Participant.User.DateCreated = DateTime.Now;
                    studentRegisterViewModel.Participant.User.Id = 3;
                    studentRegisterViewModel.Participant.User.IsApproved = false;
                    studentRegisterViewModel.Participant.User.RoleId = 2;
                    studentRegisterViewModel.Participant.CountryId = 1;
                    var interests = new List<Interest>();
                    foreach (var interestItem in studentRegisterViewModel.SelectedInterests)
                    {
                        var interest = new Interest { Id = interestItem };
                        interest.Participants.Add(studentRegisterViewModel.Participant);
                        interests.Add(interest);
                        studentRegisterViewModel.Participant.Interests.Add(interest);
                    }
                    studentRegisterViewModel.Participant.Interests = interests;
                    _participantService.CreatParticipant(studentRegisterViewModel.Participant);
                    var user = _userService.GetUser(studentRegisterViewModel.Participant.User.Username);
                }
            }
            studentRegisterViewModel.Gender =
                Enum.GetNames(typeof(Gender)).Select(
                    x => new KeyValuePair<string, string>(x, x.ToString(CultureInfo.InvariantCulture)));
            studentRegisterViewModel.Interests = _interestService.GetAllInterests();
            return View(studentRegisterViewModel);
        }

Participant Repository (DAL)

 public class ParticipantRepository : Repository<Participant>, IParticipantRepository 
    {
        public ParticipantRepository(IDatabaseFactory databaseFactory)
            : base(databaseFactory)
        {
        }
    }

Participant Service (BLL)

public class ParticipantService : IParticipantService
    {
        private readonly IParticipantRepository _participantRepository;
        private readonly IUnitOfWork _unitOfWork;

        public ParticipantService(IParticipantRepository participantRepository, IUnitOfWork unitOfWork)
        {
            this._participantRepository = participantRepository;
            this._unitOfWork = unitOfWork;
        }

        public void CreatParticipant(Participant participant)
        {
            _participantRepository.Add(participant);
            _unitOfWork.Commit();
        }
}

Database Factory

public class DatabaseFactory : Disposable, IDatabaseFactory
    {
        private STNDataContext _stnDataContext;
        public DatabaseFactory()
        {
            Database.SetInitializer<STNDataContext>(null);
        }
        public STNDataContext Get()
        {
            return _stnDataContext ?? (_stnDataContext = new STNDataContext());
        }
        protected override void DisposeCore()
        {
            if (_stnDataContext != null)
                _stnDataContext.Dispose();
        }
    }

Unit of Work Class

public class UniOfWork : IUnitOfWork
{
    private readonly IDatabaseFactory _databaseFactory;
    private STNDataContext _stnDataContext;

    public UniOfWork(IDatabaseFactory databaseFactory)
    {
        this._databaseFactory = databaseFactory;
    }

    public STNDataContext StnDataContext
    {
        get { return _stnDataContext ?? (_stnDataContext = _databaseFactory.Get()); }
    }

    public void Commit()
    {
        StnDataContext.Commit();
    }

}

When I try to Create Participant I get following error.

Cannot insert the value NULL into column 'InterestName', table 'StudyTourNetworkDB.dbo.Interests'; column does not allow nulls. INSERT fails.\r\nThe statement has been terminated.

Ideally as per my thinking it should insert Participant Information in Participants table and Participants Interests in ParticipantsInterests table. But it is trying to insert record in Interests table also which should not happen. Please help me resolve this problem. I may be doing wrong by creating many-to-many association.

Thanks

Note : I could understand the problem as Interests collection does not get added / attach to context but I could not find out how to add Interest collection to the same context with repository pattern and unit of work.

Please provide me the solutions. Thanks in advance

Answer 1

You are correct in that your Interest objects are being re-added, because the copies held in your model are not being tracked by EF and therefore it thinks they are new. Instead, you will need to look up the versions from your repository, and add those instead.

Instead of:

var interests = new List<Interest>();
foreach (var interestItem in studentRegisterViewModel.SelectedInterests)
{
    var interest = new Interest { Id = interestItem };
    interest.Participants.Add(studentRegisterViewModel.Participant);
    interests.Add(interest);
    studentRegisterViewModel.Participant.Interests.Add(interest);
}
studentRegisterViewModel.Participant.Interests = interests;

Try something like:

// Look up the actual EF entities which match your selected items.  You'll  
// probably need to adapt this to make it work
var selectedInterestIds = studentRegisterViewModel.SelectedInterests.Select(i => i.Id);
var interests = _interestService.GetAllInterests().Where(i => selectedInterestIds.Contains(i.Id));    
studentRegisterViewModel.Participant.Interests = interests;

Note that with a many-to-many relationship, you don't need to set both sides - in your example you were filling in the Participant field of the Interest entity - this will be set automatically by EF since you're adding it to the Interests property of the Participant.