复杂数据映射器查询关联

我是DataMapperORM的初学者,所以我对复杂的查询有疑问。

class User  
    property :id, Serial
    property :login, String

    has n, :actions
end

class Item
    property :id, Serial
    property :title

    has n, :actions
  has n, :users, :through => :actions
end

class Action
    property :user_id, Integer
    property :item_id, Integer

    belongs_to :item
    belongs_to :user
end

数据库中的数据是这样的:

+ ------- + + ------- + + ------- +
| Users   | | Items   | | Actions |
+ ------- + + ------- + + ------- +
| 1  | u1 | | 3  | i1 | | 1  | 4  |
| 2  | u2 | | 4  | i2 | | 1  | 3  |
| ....... | | 5  | i3 | | 1  | 4  |
+ ------- + | ....... | | 1  | 5  |
            + ------- + | 1  | 6  |
                        | 1  | 3  |
                        | ....... |
                        + ------- +

例如,用户1的结果应如下所示:

+ -------------------- |
| Items (item_id, num) |
+ -------------------- |
| 3, 2                 |
| 4, 2                 |
| 5, 1                 |
| 6, 1                 |
+ -------------------- +

P.S.符合我需要的常规SQL查询:

SELECT i.id, i.title, COUNT(*) as 'num'
FROM actions a
JOIN items i on i.id = a.item_id
WHERE a.user_id = {USERID}
GROUP by a.id
ORDER BY num DESC
LIMIT 10;

那么,如何做到这一点,是否有关于复杂datamapper查询的文档?