Mongodb aggregation $unwind then count

Question

Here is my problem : in my Mongo database, I have a collection with items like :

{
  'id': 1,
  'steps': [
    {
      action: 'start',
      info: 'foo'
    },
    {
      action: 'stop',
      info: 'bar'
    }
  ]
}

I would like to get the total number of steps 'start'. I tryed to use the MongoDB aggregation framework : I use $unwind on steps.action and $match on steps.action to match 'start'.

However, I get too much data and reach the aggregation's limit : exception: aggregation result exceeds maximum document size (16MB). I don't need the data, I just want the count, but I couldn't find how to do it (tryed with $group without success).

Thanks in advance,

Answer 1

If you want the count you can use this

db.test.count({"steps.action":"start"})

but this will not take into account if steps contain multiple steps with action start.

When you also need to count all steps with start then you need to unwind the array, make a match on steps.action and then group the results to count.

db.test.aggregate([{$unwind:"$steps"}, {$match:{"steps.action":"start"}},{ $group: { _id: null, count: { $sum: 1 } } }])