mongodb count based on array of ids

1> db.mycollection.insert({'foo':[1,2,3,4]})
2> db.mycollection.insert({'foo':[5,6,7]})
3
4> db.mycollection.aggregate([{$project: { count: { $size:"$foo" }}}])
5{ "_id" : ObjectId("5314b5c360477752b449eedf"), "count" : 4 }
6{ "_id" : ObjectId("5314b5c860477752b449eee0"), "count" : 3 }
7

1{ $size: <expression> }
2

1db.collection.aggregate([
2    { $unwind: "$connects" },
3
4    // count all occurrences
5    { "$group": { "_id": {skill: "$skill", parser_id: "$connects.parser_id"}, "count": { "$sum": 1 } }},
6
7    // sum all occurrences and count distinct
8    { "$group": { "_id": "$_id.skill", "quantity": { "$sum": 1 } }},
9
10    // (optional) rename the '_id' attribute to 'skill'
11    { $project: { 'skill': '$_id', 'quantity': 1, _id: 0 } }
12])
13