Mongodb Aggregation count array/set size

Question

Here\'s my problem:

Model:

{ application: \"abc\", date: Time.now, status: \"1\" user_id: [ id1, id2, id4] }

{ application: \"abc\",

Answer 1

The following will return number of uniqueUsers per application. This will apply an group operation to a result of a group operation by using pipeline feature of mongodb.

{ $match: { application: "abc" } }, 
{ $unwind: "$users" }, 
{ $group: { _id: "$status", users: { $addToSet: "$users" } } }, 
{ $unwind:"$users" }, 
{ $group : {_id : "$_id", count : {$sum : 1} } }

Hopefully this will be done in an easier way in the following releases of mongo by a command which gives the size of an array under a projection. {$project: {id: "$_id", count: {$size: "$uniqueUsers"}}} https://jira.mongodb.org/browse/SERVER-4899

Cheers

Answer 2

Sorry I'm a little late to the party. Simply grouping on the 'user_id' and counting the result with a trivial group works just fine and doesn't run into doc size limits.

[
    {$match: {application: 'abc', date: {$gte: startDate, $lte: endDate}}},
    {$unwind: '$user_id'},
    {$group: {_id: '$user_id'}},
    {$group: {_id: 'singleton', count: {$sum: 1}}}
];

Answer 3

Use $size to get the size of set.

[
    {
        $match: {"application": "abc"}
    },
    {
        $unwind: "$user_id"
    },
    {
        $group: {
            "_id": "$status",
            "application": "$application",
            "unique_user_id": {$addToSet: "$user_id"}
        }
    },
    {
        $project:{
            "_id": "$_id",
            "application": "$application",
            "count": {$size: "$unique_user_id"}
        }
    }
]