mongodb count sub-document and list totals

Question

In mysql, I have a query like:

mysql> SELECT user_id, count(user_id) as dup FROM addressbook GROUP BY user_id HAVING dup>20 ORDER BY dup;

Answer 1

You can directly get the number of elements in the addressBook array field of each user by using $size:

db.users.aggregate([
    {$project: {_id: 1, count: {$size: '$addressBook'}}}
])

Output:

{
    "result" : [ 
        {
            "_id" : ObjectId("540c83f9d901f28b921a328c"),
            "count" : 2
        }
    ],
    "ok" : 1
}

Note that the $size operator was introduced in MongodB 2.6.

Answer 2

Your id condition is blank and you do not have a where clause. I think this would be more like your sql query. Note that in case you have user_id in your users collection then you should replace $_id with $user_id

db.users.aggregate( [

{ $match: { dub: { $gt: 20 } } },
 {
  $group: {
     _id: "$_id",
     count: { $sum: 1 }
   }
 }
])