假设我的MongoDB模式如下所示:
{car_id: "...", owner_id: "..."}
这是一种多对多的关系.例如,数据可能如下所示:
+-----+----------+--------+
| _id | owner_id | car_id |
+-----+----------+--------+
| 1 | 1 | 1 |
| 2 | 1 | 2 |
| 3 | 1 | 3 |
| 4 | 2 | 1 |
| 5 | 2 | 2 |
| 6 | 3 | 4 |
| 7 | 3 | 5 |
| 8 | 3 | 6 |
| 9 | 3 | 7 |
| 10 | 1 | 1 | <-- not unique
+-----+----------+--------+
我想知道每个车主拥有的汽车数量.在SQL中,这可能看起来像:
SELECT owner_id, COUNT(*) AS cars_owned
FROM (SELECT owner_id FROM car_owners GROUP BY owner_id, car_id) AS t
GROUP BY owner_id;
在本例中,结果如下所示:
+----------+------------+
| owner_id | cars_owned |
+----------+------------+
| 1 | 3 |
| 2 | 2 |
| 3 | 4 |
+----------+------------+
我如何使用MongoDB和聚合框架来完成同样的事情?