Fix: Give identificator 

If I want to give to __construct array with ID (so as not to tripped autoincrement)
Example: $user = new User(array('id' => 1, 'password' => '777'));
'user_name' - is primary key and identificator
Without fix: $user->save() - will not work

If I want to change id on row:
$user = new User(1); $user->load();
$user->user_name = 2;
$user->save(); - will not work

With fix: will be work.