Let's say my MongoDB schema looks like this:

{car_id: "...", owner_id: "..."}

This is a many-to-many relationship. For example, the data might look like this:

+-----+----------+--------+
| _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
+-----+----------+--------+

I want to get the number of cars owned by each owner. In SQL, this might look like:

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;

In this case, the result would look like this:

+----------+------------+
| owner_id | cars_owned |
+----------+------------+
|        1 |          3 |
|        2 |          2 |
|        3 |          4 |
+----------+------------+

How can I accomplish this same thing using MongoDB using the aggregation framework?

mongodb aggregation-framework
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