Сущности многие ко многим: Книги Авторы, сложный запрос: перечисление авторов каждой книги

AuthorTable.php

namespace Orm;

use Bitrix\Main\Entity;

class AuthorTable extends Entity\DataManager{
    public static function getTableName()
    {
        return 'author';
    }

    public static function getMap()
    {
        return array(
            new Entity\IntegerField('ID', array(
                'primary' => true,
                'autocomplete' => true
            )),
            new Entity\StringField('NAME')
        );
    }
}

BookAuthorTable.php

namespace Orm;

use Bitrix\Main\Entity;
use Bitrix\Main\Type\Date;

class BookAuthorTable extends Entity\DataManager{
    public static function getTableName()
    {
        return 'book_author';
    }

    public static function getMap()
    {
        return array(
            new Entity\IntegerField('ID', array(
                'primary' => true,
                'autocomplete' => true
            )),
            new Entity\IntegerField('BOOK_ID', array(
                'primary' => true
            )),
            new Entity\ReferenceField(
                'BOOK',
                'Orm\BookTable',
                array('=this.BOOK_ID' => 'ref.ID')
            ),
            new Entity\IntegerField('AUTHOR_ID', array(
                'primary' => true
            )),
            new Entity\ReferenceField(
                'AUTHOR',
                'Orm\AuthorTable',
                array('=this.AUTHOR_ID' => 'ref.ID')
            )
        );
    }
}

BookTable.php

namespace Orm;

use Bitrix\Main\Entity;
use Bitrix\Main\Type\Date;

class BookTable extends Entity\DataManager{
    public static function getTableName()
    {
        return 'book';
    }

    public static function getMap()
    {
        return array(
            new Entity\IntegerField('ID', array(
                'primary' => true,
                'autocomplete' => true
            )),
            new Entity\IntegerField('ISBN'),
            new Entity\StringField('TITLE'),
            new Entity\DateField('PUBLISH_DATE', [
                'default_value' => new Date
            ])
        );
    }
}

orm entities and joins.txt

 $res = BookTable::getList(array(
        'select' => array(
            'TITLE',
            'AUTHORS',
        ),
        'group' => array('Orm\BookAuthor:BOOK.BOOK_ID'),
        'runtime' => array(
            new Entity\ExpressionField('AUTHORS', 'GROUP_CONCAT(%s SEPARATOR \', \' )', ["Orm\BookAuthor:BOOK.AUTHOR.NAME"]),
            new Entity\ExpressionField('CNT', "COUNT(DISTINCT %s)", ["Orm\BookAuthor:BOOK.AUTHOR.NAME"])
        ),
        'filter' => array('>=CNT' => 2)
    ));


    while ($row = $res->fetch())
    {
        $rows[] = $row;
    }