ID of newly created entity not getting written into view

I have an action that basically does this:

public ViewResult Save(Foo postedFoo)           
{
  Foo foo;
  if (postedFoo.Id == 0) //New foo; need to save for first time
  {
    foo = new Foo();
  }
  else //Already exists, need to load and update
  {
    foo = FooRepository.LoadFoo(postedFoo.Id);
  }
  UpdateModel(foo);
  FooRepository.Save(foo);
  return View(foo);
}

As you can see, the action handles both creating new Foo instances and updating existing ones.

The Foo's Id property is written into a hidden field in the view like this:开发者_开发技巧

@Html.HiddenFor(m => m.Id)

The problem is that in the scenario where the user saves a new Foo, the Foo's Id property (which is being set in the line FooRepository.Save(foo)) is NOT being written to the hidden field before the page is redisplayed to the user.

This means that if the user saves a new Foo, then immediately changes something and saves again, the controller thinks it's another new Foo and creates a new Foo in the database rather than just updating it.

Can anyone suggest why the hidden field is not being populated?

Write

ModelState.Remove("Id")

Before returning View();

This behavior is caused by the fact that ModelState is the primary supplier for values when rendering (yes, not the Model itself). So removing Id from ModelState, makes editors use Model's value (In your case, updated Id)

shouldn't the else line read

foo = FooRepository.LoadFoo(postedFoo.Id);

Also, the new Foo(); isn't setting the value of ID anywhere unless you've hardcoded it into your model. But if FooRepository.Save(foo) is meant to do this, it's not. You need to return the ID field back to your controller.

foo = FooRepository.Save(foo);