MongoDB: count the number of items in an array

In MongoDB 2.6, the Aggregation Framework has a new array $size operator you can use:

> db.mycollection.insert({'foo':[1,2,3,4]})> db.mycollection.insert({'foo':[5,6,7]})> db.mycollection.aggregate([{$project: { count: { $size:"$foo" }}}]){ "_id" : ObjectId("5314b5c360477752b449eedf"), "count" : 4 }{ "_id" : ObjectId("5314b5c860477752b449eee0"), "count" : 3 }

if you are on a recent version of mongo (2.2 and later) you can use the aggregation framework.

db.mycollection.aggregate([  {$unwind: '$foo'},  {$group: {_id: '$_id', 'sum': { $sum: 1}}},  {$group: {_id: null, total_sum: {'$sum': '$sum'}}}])

which will give you the total foos of your collection.

Omitting the last group will aggregate results per record.

Using Projections and Groups

db.mycollection.aggregate(    [        {            $project: {                _id:0,                foo_count:{$size:"$foo"},            }        },         {            $group: {                foo_total:{$sum:"$foo_count"},            }        }    ])

Multiple child array counts can also be calculated this way

db.mycollection.aggregate(    [        {            $project: {                _id:0,                foo1_count:{$size:"$foo1"},                foo2_count:{$size:"$foo2"},            }        },         {            $group: {                foo1_total:{$sum:"$foo1_count"},                foo2_total:{$sum:"$foo2_count"},            }        }    ])