MongoDB: count the number of items in an array - mongodb

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 the MongoDB shell to count how many instances are within foo? something like:
db.mycollection.foos.count() or db.mycollection.foos.size()?
Each document in the array needs to have a unique foo_id and I want to do a quick count to make sure that the right amount of elements are inside of an array for a random document in the collection.

Answer 1

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 }

Answer 2

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.

Answer 3

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"},
}
}
]
)