Laravel 5.1 eloquent tries to INSERT into DB instead of UPDATE
You could also do:
$m = $m->whereField($field)->firstOrFail();
//catch exception... (log or something else)
$m->anotherField = $field;$m->save();
Doing this will let you catch the exception so you can log and display an error page as necessary. To catch the ModelNotFoundException, add some logic to your app/Exceptions/Handler.php file.