问SubmitChanges并不总是更新数据库

EN

public static bool changeFacebookIDByEmail(string email, string facebookID)
{
    UserProfile profile = (from s in _dc.Users
                           join u in _dc.Memberships on s.UserId equals u.UserId
                           join i in _dc.UserProfiles on u.UserId equals i.userID
                           where u.Email == email
                           select i).SingleOrDefault();

    profile.facebookID = facebookID;

    ChangeSet cs = _dc.GetChangeSet();

    _dc.SubmitChanges();

    if (cs.Updates.Count <= 0)
        return false;
    else
        return true;
}