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

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.