How to update related entities in Entity Framework

I have an MVC project and using Entity Framework Code First and POCO objects for the database. For example:

public class ClassA
{
  public int? Id {get; set;}
  public string Name { get; set;}
  public virtual ClassB B {get; set;}
}

public class ClassB
{
  public int? Id {get;set;}
  public string Description {get;set;}
}

I have an ActionResult that create or edit a model. The problem is when I call this ActionResult to update the model, and model.B has been changed, the relation is not saved in the database. When the ActionResult is called to create a new object it works as expected. How do I solve this?

public ActionResult Save(ClassA model)
{
  model.B = GetBFromDb(model.B.Id.Value);

  if(ModelState.IsValid)
  {
    if (id.HasValue)
    {
      context.Entry(model).State = System.Data.EntityState.Modified;
    }
    else
    {
      context.ClassAs.Add(model);
    }
    context.SaveChanges();
    // redirect to index etc.
  }
  return View("EditForm", model);
}

---

I solved it by changing ClassA with a foreign key to ClassB , and setting the value of BId :

public class ClassA
{
  public int? Id {get; set;}
  public string Name { get; set;}
  public int BId {get;set;} // foreign key to B
  public virtual ClassB B {get; set;}
}

Why does this foreign key property does the job instead of the generated foreign ky of EF?

---

You can't simply call:

context.Entry(model).State = System.Data.EntityState.Modified;

The entity has to be retrieved from the context first so that EF can begin tracking it. Then you'll want to apply any changes to that entity before calling context.SaveChanges() .

var entity = context.ClassAs.Find(model.Id);

// set properties you want to modify on entity
entity.Name = model.Name;
entity.ClassB = context.ClassBs.Find(model.ClassB.Id);
// other changes to entity as required...

context.SaveChanges();

This way EF is tracking entity and knows to apply an update against it.

---

If you attach and then set the state to modified entity framework will send all properties for update. You don't have to take the otter approach here below as that causes a whole separate load to occur. If you do it that way theres no sense in passing in a model, only an id and then you call TryUpdateModel to fill in the properties from the form.

Personally the attach as modified here is a bit cleaner as it doesn't require a complete round trip because of loading data

http://geekswithblogs.net/michelotti/archive/2009/11/27/attaching-modified-entities-in-ef-4.aspx

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