Mongodb 聚合 $unwind 然后计数

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 } } }])

Answer 2

try this

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

Answer 3

在 mongodb 聚合框架中，管道阶段的大小限制最大为 100MB，而它提供的结果是 BSON 文件或集合文档的最大大小为 16MB 所以你可以$match仅在需要条件和$group它以便仅输出小于 16MB 的所需结果。

Answer 4

You may not need aggregation for this simple query. See below code.

for (var i = 10000; i >= 0; i--) {
    var a = {
      'id': 1,
      'steps': [
        {
          action: 'start',
          info: 'foo'
        },
        {
          action: 'stop',
          info: 'bar'
        }
      ]
    }

    a.id = i;
    var rnd = Math.floor((Math.random() * 3) + 1);
    if (rnd == 1)
    {
        a.steps[0].action = 'none';
    }
    if (rnd == 2)
    {
        a.steps.push({ action: 'start', info: 'foo' })

    }
    db.obj.insert(a);
};

This code creates random number of actions. If you need only number of documents which contains action: 'start' then below query.

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

I get following count in my run.

> db.obj.count({"steps.action":"start"})
6756

But if you need number of {action: 'start'} in the documents then aggregation query needed. You unwind then match

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

This outputs:

{ "_id" : null, "count" : 10054 }



if you get your exception again use **allowDiskUse : true** option. See [here][1].

db.obj.aggregate(
[
 ....
]
 ,
 { 
   allowDiskUse : true
 }

)