i'm using Entity Framework 4.0 and having a silly problem that i can't figure out.

I have two tables:

1. Contact: Id (primary key), Value, ContactTypeId (foreign key to ContactType)
2. ContactType: Id (Primary key), Type (Home, Cell, Work, etc.)

Entity Framework created the following two entities:

1. Contact: Id, Value, ContactType (Navigation Property)
2. ContactType: Id, Type, Contact (Navigation Property)

I'm using the following code to get the contact and update the contact type for that particular contact:

```
Contact contact = dbContext.Contacts.Single(c => c.Id == 12345);
contact.ContactType.Id = 3;

```

Throws the following exception: 

```
The property 'Id' is part of the object's key information and cannot be modified.

```

It looks so simple! I don't get it!