has_many Example
=================
```php
/*** TestPage.php ***/
use SilverStripe\Forms\GridField\GridField;
use SilverStripe\Forms\GridField\GridFieldConfig_RecordEditor;
use UndefinedOffset\SortableGridField\Forms\GridFieldSortableRows;

class TestPage extends Page {
	private static $has_many=array(
		'TestObjects'=>'TestObject'
	);

	public function getCMSFields() {
		$fields=parent::getCMSFields();

		$conf=GridFieldConfig_RecordEditor::create(10);
		$conf->addComponent(new GridFieldSortableRows('SortOrder'));

		$fields->addFieldToTab('Root.TestObjects', new GridField('TestObjects', 'TestObjects', $this->TestObjects(), $conf));

		return $fields;
	}
}


/*** TestObject.php ***/
use SilverStripe\ORM\DataObject;

class TestObject extends DataObject {
	private static $db=array(
		'Title'=>'Text',
		'SortOrder'=>'Int'
	);

    private static $has_one=array(
        'Parent'=>'TestPage'
    );

	private static $default_sort='SortOrder';
}
```