需要了解CakePHP中finderQuery的用法

Question

$this->Post->bindModel
        (
          array
          (
            'belongsTo' => array
                           (
                             'Person' => array
                                          (
                                            'className' => 'Person',
                                            'foreignKey' => 'person_id'
                                          )
                           ),
            'hasMany' => array
                           (
                             'PostGroup' => array
                                            (
                                              'className' => 'PostGroup',
                                              'foreignKey' => 'post_id',

                                              'conditions' => array('PostGroup.status' => 'active'),

                                              'finderQuery' => 'SELECT count(PostGroup.id) as no_of_groups, PostGroup.post_id
                                                               from post_groups as PostGroup
                                                               WHERE PostGroup.post_id in ({$__cakeID__$})
                                                               GROUP BY PostGroup.post_id'
                                            )
                           )
          )
        );

有人可以向我解释finderQuery在这里做了什么吗？我已经按照Cake文档进行了操作，但不知怎的，这里无法解释。 TIA。