PHP ActiveRecord - find_by_sql + include

我需要使用带有PHP ActiveRecord的自定义查询，因为我需要UNION语句，即使我在两个查询中都查询同一个表。我希望能够在结果中包括我在模型上作为belongs_to的列。

这是我的型号代码：

class FeedActivity extends MyModel {
    static $table_name = 'feed_activity';
    static $belongs_to = array(
        array('user_reference', 'class_name' => 'User', 'foreign_key' => 'user_reference_id'),
        array('follow', 'class_name' => 'Follow', 'foreign_key' => 'follow_id'),
        array('challenge_user', 'class_name' => 'ChallengeUser', 'foreign_key' => 'challenge_user_id'),
    );
}

我的问题是：

$list = FeedActivity::find_by_sql('
    (
        SELECT
            *
        FROM
            feed_activity
        WHERE
            challenge_user_id IS NOT NULL
    )
    UNION ALL
    (
        SELECT
            feed_activity.*
        FROM
            feed_activity
        JOIN
            follow ON follow.id_follower = ' .(int) $user_id . '
                    AND follow.id = feed_activity.follow_id
        WHERE
            follow_id IS NOT NULL
        LIMIT 0, 5
    )
    ORDER BY created_at DESC
    LIMIT 0, 10
');

有没有想过我该怎么做？

感谢