Entity Framework throwing error when updating records - asp.net-mvc-3

Question

Store update, insert, or delete statement affected an unexpected number of rows (0). Entities may have been modified or deleted since entities were loaded. Refresh ObjectStateManager entries.
I get this error, but I am the only person using the database. I am using Entity Framework 4.1 with DBContext.
I am updating my records and SQL Profiler is showing a queue being sent in. What could be the causes of this issue?
The post:
[HttpPost]
public ActionResult EditUser(User user)
{
uow.UserRepository.Update(user);
uow.Save();
return RedirectToAction("Index", "User");
}
On this call:
public void Save()
{
_context.SaveChanges();
}
This is how it is attached
public virtual void Update(TEntity entityToUpdate)
{
dbSet.Attach(entityToUpdate);
context.Entry(entityToUpdate).State = EntityState.Modified;
}
Update:
public class UnitOfWork : IDisposable
{
private StudentSchedulingEntities _context = new StudentSchedulingEntities();
private GenericRepository<User> userRepository;
private GenericRepository<UserRole> userRoleRepository;
private bool disposed = false;
public GenericRepository<User> UserRepository
{
get
{
if (this.userRepository == null)
{
this.userRepository = new GenericRepository<User>(_context);
}
return userRepository;
}
}
public GenericRepository<UserRole> UserRoleRepository
{
get
{
if (this.userRoleRepository == null)
{
this.userRoleRepository = new GenericRepository<UserRole>(_context);
}
return userRoleRepository;
}
}
public void Save()
{
_context.SaveChanges();
}
protected virtual void Dispose(bool disposing)
{
if (!this.disposed)
{
if (disposing)
{
_context.Dispose();
}
}
this.disposed = true;
}
public void Dispose()
{
Dispose(true);
GC.SuppressFinalize(this);
}
}

Answer 1

The ID field must be there in order to update the information properly. Otherwise, it will be throw a null. (I forgot to put the hidden field for ID in)