MongoDB: count the number of items in an array

Question

I have a collection where every document in the collection has an array named foo that contains a set of embedded documents. Is there currently a trivial way in

Answer 1

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"},
            }
        }
    ]
)