Question

I have three tables that I am working with User, Application, ApplicationAdministrator. ApplicationAdministrator is a mapping table to link User to Application which has a many-to-many relationship. I get the following error when I try to save off a new Application with a User added as an Administrator:

The relationship between the two objects cannot be defined because they are attached to different ObjectContext objects.

So my next step was to create a BaseRepository that has a common context to pull from. However, now I get the following error when I try to modify an entity that is already attached to the context:

An object with the same key already exists in the ObjectStateManager. The ObjectStateManager cannot track multiple objects with the same key.

Why is this such a difficult process? I have seen the solutions to attach and reattach and detach and spin around on your head 5 times and then everything will work. Attaching the entities to one context ends up duplication one of the entities depending on which context I attach it to.

All help is greatly appreciated!

UserRepository.cs:

public class UserRepository : BaseRepository<User>, IUserRepository
{
    // private ManagerDbContext _context = new ManagerDbContext();

    public UserRepository(ManagerDbContext context)
        : base(context) { }

    public IQueryable<User> Users
    {
        get { return _context.Users.Include("Administrates").Include("Company"); }
    }

    public void SaveUser(User user)
    {
        _context.Entry(user).State = user.Id == 0 ? EntityState.Added : EntityState.Modified;

        _context.SaveChanges();
    }

    public void DeleteUser(User user)
    {
        _context.Users.Remove(user);

        _context.SaveChanges();
    }
}

ApplicationRepository.cs:

public class ApplicationRepository : BaseRepository<Application>, IApplicationRepository
{
    // private ManagerDbContext _context = new ManagerDbContext();

    public ApplicationRepository(ManagerDbContext context)
        : base(context) { }

    public IQueryable<Application> Applications
    {
        get { return _context.Applications.Include("Administrators"); }
    }

    public void SaveApplication(Application app)
    {
        _context.Entry(app).State = app.Id == 0 ? EntityState.Added : EntityState.Modified;
        _context.SaveChanges();
    }

    public void DeleteApplication(Application app)
    {
        _context.Applications.Remove(app);
        _context.SaveChanges();
    }
}

UserConfiguration.cs:

public UserConfiguration()
{
    this.HasKey(x => x.Id);

    this.Property(x => x.Id).HasDatabaseGeneratedOption(DatabaseGeneratedOption.Identity);
    this.Property(x => x.FirstName).IsRequired();
    this.Property(x => x.LastName).IsRequired();
    this.Property(x => x.Username).IsRequired();
    this.Property(x => x.CompanyId).IsRequired();

    this.HasRequired(user => user.Company).WithMany().HasForeignKey(user => user.CompanyId);
    this.HasRequired(user => user.Company).WithMany(company => company.Users).WillCascadeOnDelete(false);

    this.HasMany(user => user.Administrates)
        .WithMany(application => application.Administrators)
        .Map(map => map.MapLeftKey("UserId")
            .MapRightKey("ApplicationId")
            .ToTable("ApplicationAdministrators"));
}

ApplicationConfiguration.cs:

public ApplicationConfiguration()
{
    this.HasKey(x => x.Id);

    this.Property(x => x.Id).HasDatabaseGeneratedOption(DatabaseGeneratedOption.Identity);
    this.Property(x => x.Name).IsRequired();
    this.Property(x => x.Description);

    this.HasMany(application => application.Administrators)
        .WithMany(user => user.Administrates)
        .Map(map => map.MapLeftKey("ApplicationId")
            .MapRightKey("UserId")
            .ToTable("ApplicationAdministrators"));
}

Snippet for saving the entities.

long appId = Int64.Parse(form["ApplicationId"]);
long userId = Int64.Parse(form["UserId"]);

Application app = appRepository.Applications.FirstOrDefault(a => a.Id == appId);
User user = userRepository.Users.FirstOrDefault(u => u.Id == userId);

app.Administrators.Add(user);

appRepository.SaveApplication(app);

return RedirectToAction("Index");

Answer 1

Here is my solution that I finally came up with.

I created a Func dictionary in order to attach an entity to the correct EntitySet in my context. The one downfall is that you have to hard code the EntitySet name some, so I did it in a static variable within my POCO.

BaseRepository.cs

public class BaseRepository<T> where T : class 
{
    public static ManagerDbContext baseContext;

    public BaseRepository() { }

    public BaseRepository(ManagerDbContext context)
    {
        baseContext = context;
    }

    private static object _entity;

    public void AttachEntity(object entity)
    {
        _entity = entity;

        entityAttachFunctions[entity.GetType().BaseType]();
    }

    private Dictionary<Type, Func<bool>> entityAttachFunctions = new Dictionary<Type, Func<bool>>()
    {
        {typeof(User), () => AttachUser()},
        {typeof(Application), () => AttachApplication()}
    };

    private static bool AttachUser()
    {
        ((IObjectContextAdapter)baseContext).ObjectContext.AttachTo(User.TableName, _entity);

        return true;
    }

    private static bool AttachApplication()
    {
        ((IObjectContextAdapter)baseContext).ObjectContext.AttachTo(Application.TableName, _entity);

        return true;
    }
}

UserRepository.cs

public void AttachEntity(object entity)
{
    baseContext = _context;

    base.AttachEntity(entity);
}

public void DetachUser(User user)
{
    _context.Entry(user).State = EntityState.Detached;

    _context.SaveChanges();
}

ApplicationRepository.cs

public void AttachEntity(object entity)
{
    baseContext = _context;

    base.AttachEntity(entity);
}

public void DetachApplication(Application app)
{
    _context.Entry(app).State = EntityState.Detached;

    _context.SaveChanges();
}

AdminController.cs

long appId = Int64.Parse(form["ApplicationId"]);
long userId = Int64.Parse(form["UserId"]);

Application app = appRepository.Applications.FirstOrDefault(a => a.Id == appId);
User user = userRepository.Users.FirstOrDefault(u => u.Id == userId);

userRepository.DetachUser(user);

appRepository.AttachEntity(user);

app.Administrators.Add(user);

appRepository.SaveApplication(app);