SilverStripe单一对一对象bug

Question

I have a one-to-one relation between Team and Coach. I use has one edit module to edit the Coach's name directly via the Team gridfield edit page.

On the first Team save, two Coach records are created with the supplied names. However on each consecutive save a new Coach record is created, which is obviously not the desired outcome. Instead, it should save the updated names to the same records, because the coaches don't change, it's just their columns (names) that change.

Team

class Team extends DataObject {

    private static $has_one {
        'MainCoach' => 'Coach',
        'AssistentCoach' => 'Coach'
    );
}

Coach

class Coach extends DataObject {

    private static $db = array(
        'Name' => 'Varchar'
    );

Team edit page

public function getCMSFields() {

    TextField::create('MainCoach-_1_-Name', 'Main coach name'),      // main coach name
    TextField::create('AssistentCoach-_1_-Name', 'Assistent coach name')  // assistent coach name

}