新实体中的身份用户

Question

I am trying to make an ASP.NET MVC Project. I have a forum controller with a SubmitPost action. Here is the code for the action:

[HttpPost]
[Authorize]
[ValidateAntiForgeryToken]
public ActionResult Submit(PostSubmitViewModel model)
{
    if (!this.ModelState.IsValid)
    {
        return View(model);
    }

    var category = this.categoryService.GetById(model.CategoryId);
    var user = this.userManager.FindById(this.User.Identity.GetUserId());

    this.postService.Submit(new Post
    {
        Title = model.Title,
        Content = model.Content,
        Category = category,
        Author = user,
    });
    return RedirectToAction("Category", "Forum", new { id =  model.CategoryId });
}

As you can see there is a postService and userManager . They are injected by Ninject and they both use the same DbContext - per request scope. Login, Register all work as expected.

private static void RegisterServices(IKernel kernel)
{
    kernel.Bind<ApplicationDbContext>().ToSelf().InRequestScope();

    // Identity bindings
    kernel.Bind<IUserStore<ApplicationUser>>().To<UserStore<ApplicationUser>>().InRequestScope()
        .WithConstructorArgument<DbContext>(kernel.Get<ApplicationDbContext>());
    kernel.Bind<IAuthenticationManager>().ToMethod(c => HttpContext.Current.GetOwinContext().Authentication).InRequestScope();
    kernel.Bind<ApplicationUserManager>().ToSelf().InRequestScope();
    kernel.Bind<ApplicationSignInManager>().ToSelf().InRequestScope();
}

The post service does only this:

public void Submit(Post post)
{
    this.context.Posts.Add(post);
    this.context.SaveChanges();
}

My problem is that when the action is called i get an exception:

An entity object cannot be referenced by multiple instances of IEntityChangeTracker.

at this line:

this.context.Posts.Add(post);

I'm pretty sure i have only one dbContext instance for both the service and userManager. When I insert the post with a null author it works fine. I guess EF is trying to save the existing user again but I don't get the same error with the category that also comes from an external service and already exists. I tried putting a breakpoint on MyDbContext 's constructor and it's only called once. Maybe there is something specific about UserManager that I don't know about?

Here is my user manager class:

public class ApplicationUserManager : UserManager<ApplicationUser>
{
    public ApplicationUserManager(IUserStore<ApplicationUser> store)
        : base(store)
    {
        // Validation configs etc.

I have no idea what exactly is causing the exception so any ideas would be helpful.

Answer 1

With limited access to the code I can only suggest:

on the object Post create a property AuthorId

Then change

var user = this.userManager.FindById(this.User.Identity.GetUserId());

this.postService.Submit(new Post
{
    Title = model.Title,
    Content = model.Content,
    Category = category,
    Author = user,
});

to

var userId = this.User.Identity.GetUserId();

this.postService.Submit(new Post
{
    Title = model.Title,
    Content = model.Content,
    Category = category,
    AuthorId = userId,
});

If the above doesn't work, on the class Post decorate the property Author with ForeignKey(nameof(AuthorId))

this is all i can give at the moment without more information. I hope it helps