Entity framework 4, update specific properties

I'm used to using code first where I can do:

db.Users.Attach(user)开发者_开发技巧;
db.Entry(user).Property(propertyName).IsModified = true;

How do I do this with a DataModel approach?

I do not have the Entry method on my datacontext.

---

Use:

context.Users.Attach(user);
ObjectStateEntry entry = context.ObjectStateManager.GetObjectStateEntry(user);
entry.SetModifiedProperty(propertyName);