EF code first MVC4 - ArgumentNullException on edit - what am I doing wrong here? - entity-framework

Question

Let's say I have 3 models:
[Table("UserProfile")]
public class UserProfile //this is a standard class from MVC4 Internet template
{
[Key]
[DatabaseGeneratedAttribute(DatabaseGeneratedOption.Identity)]
public int UserId { get; set; }
public string UserName { get; set; }
}
public class Category
{
public int Id { get; set; }
public string Name { get; set; }
public virtual ICollection<Post> Posts { get; set; }
}
public class Post
{
public int CategoryId { get; set; }
public virtual Category Category { get; set; }
public int UserProfileId { get; set; }
[ForeignKey("UserProfileId")]
public virtual UserProfile UserProfile { get; set; }
}
Now, I'm trying to edit Post
[HttpPost]
public ActionResult Edit(Post post)
{
post.UserProfileId = context.UserProfile.Where(p => p.UserName == User.Identity.Name).Select(p => p.UserId).FirstOrDefault();
//I have to populate post.Category manually
//post.Category = context.Category.Where(p => p.Id == post.CategoryId).Select(p => p).FirstOrDefault();
if (ModelState.IsValid)
{
context.Entry(post.Category).State = EntityState.Modified; //Exception
context.Entry(post.UserProfile).State = EntityState.Modified;
context.Entry(post).State = EntityState.Modified;
context.SaveChanges();
return RedirectToAction("Index");
}
return View(post);
}
And I'm getting ArgumentNullException.
Quick look into debug and I can tell that my Category is null, although CategoryId is set to proper value.
That commented out, nasty-looking trick solves this problem, but I suppose it shouldn't be there at all. So the question is how to solve it properly.
I would say it's something with EF lazy-loading, beacuse I have very similar code for adding Post and in debug there is same scenerio: proper CategoryId, Category is null and despite of that EF automagically resolves that Post <-> Category dependency, I don't have to use any additional tricks.
On edit method, EF has some problem with it, but I cannot figure out what I'm doing wrong.

Answer 1

This is working as intended. Your Post object is not attached to the Context, so it has no reason to do any lazy loading. Is this the full code? I don't understand why you need to set Category as Modified since you're not actually changing anything about it.
Anyway, I recommend you query for the existing post from the Database and assign the relevant fields you want to let the user modify, like such:
[HttpPost]
public ActionResult Edit(Post post)
{
var existingPost = context.Posts
.Where(p => p.Id == post.Id)
.SingleOrDetault();
if (existingPost == null)
throw new HttpException(); // Or whatever you wanna do, since the user send you a bad post ID
if (ModelState.IsValid)
{
// Now assign the values the user is allowed to change
existingPost.SomeProperty = post.SomeProperty;
context.SaveChanges();
return RedirectToAction("Index");
}
return View(post);
}
This way you also make sure that the post the user is trying to edit actually exists. Just because you received some parameters to your Action, doesn't mean they're valid or that the post's Id is real. For example, some ill intended user could decide to edit posts he's not allowed to edit. You need to check for this sort of thing.
UPDATE
On a side note, you can also avoid manually querying for the current user's Id. If you're using Simple Membership you can get the current user's id with WebSecurity.CurrentUserId.
If you're using Forms Authentication you can do Membership.GetUser().ProviderUserKey.