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 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.