Cakephp3の相関サブクエリの記載方法

Question

Cakephp3+Mysqlの環境で、Cakephpから相関サブクエリを実行する際、結果がnullになります。

発行したいSQL

SELECT main_follows.follower_shop AS `id`, 
(SELECT (AVG(sub_follows.rating)) AS `rating` FROM follows sub_follows 
 WHERE sub_follows.follower_shop = main_follows.follower_shop GROUP BY sub_follows.follower_shop ) 
AS `average_all` FROM follows main_follows WHERE main_follows.follow = 1

--結果
--"id": "1", "average_all": 2.6641
--"id": "1", "average_all": 3.2215
--"id": "1", "average_all": 3.5427

CakePHPのコード（結果はaverage_allがnullになる）

$FollowsTable = TableRegistry::getTableLocator()->get('follows');
//メインクエリ
$main_query = $FollowsTable->find()->from(['main_follows'=>'follows'])
 ->where(['main_follows.follow'=>'1'])
 ->select(['id'=>'main_follows.follower_shop']);

//サブクエリ
$sub_query = $FollowsTable->find()->from(['sub_follows'=>'follows'])
 ->where(['sub_follows.follower_shop'=>'main_follows.follower_shop']); //①
$sub_query = $sub_query->select(['rating' => $sub_query->func()->avg('sub_follows.rating')])
 ->group(['sub_follows.follower_shop']);

$main_query = $main_query->select(['average_all' => $sub_query]);

//結果
//"id": "1", "average_all": null
//"id": "1", "average_all": null
//"id": "1", "average_all": null

①の'main_follows.follower_shop'がメインクエリから値を取得できていないように思えますが、原因がわかりません。
よろしくお願いします。

Answer 1

①をこうすれば出来ました。
->where(['sub_follows.follower_shop = main_follows.follower_shop']);