$sum

旋转门转动总次数

sensor_data.json 是某城市建筑的大门旋转次数一个月内的记录数据，记录数据每小时记录一次，一天记录 24 次。

> db.data.find().count()
31

> db.data.findOne()
{
	"_id" : ObjectId("5b3c0d4a55593f76b5a9417f"),
	"day" : 0,
	"turnstile" : [
		{
			"hour" : 0,
			"count" : 314
		},
		{
			"hour" : 1,
			"count" : "XFERR1101"
		},
		{
			"hour" : 2,
			"count" : "XFERR1101"
		},
		{
			"hour" : 3,
			"count" : 395
		},
		{
			"hour" : 4,
			"count" : 751
		},
		{
			"hour" : 5,
			"count" : 184
		},
		{
			"hour" : 6,
			"count" : 891
		},
		{
			"hour" : 7,
			"count" : 243
		},
		{
			"hour" : 8,
			"count" : 344
		},
		{
			"hour" : 9,
			"count" : 253
		},
		{
			"hour" : 10,
			"count" : 351
		},
		{
			"hour" : 11,
			"count" : 438
		},
		{
			"hour" : 12,
			"count" : 868.6595978655923
		},
		{
			"hour" : 13,
			"error" : 631
		},
		{
			"hour" : 14,
			"count" : 456
		},
		{
			"hour" : 15,
			"count" : 929
		},
		{
			"hour" : 16,
			"count" : 509
		},
		{
			"hour" : 17,
			"error" : "XFERR1101"
		},
		{
			"hour" : 18,
			"error" : 759.1292282785604
		},
		{
			"hour" : 19,
			"count" : 883
		},
		{
			"hour" : 20,
			"count" : 945.4712973582698
		},
		{
			"hour" : 21,
			"count" : 409
		},
		{
			"hour" : 22,
			"count" : "XFERR1101"
		},
		{
			"hour" : 23,
			"count" : 205
		}
	]
}

目的

计算中 count 的总数，且需要忽略错误的字段。

> db.data.aggregate([
  {
    "$unwind": "$turnstile"
  },
  {
    "$group": {
      "_id": 0,
      "sum": {
        "$sum": {
          "$convert": {
            "input": "$turnstile.count",
            "to": "int",
            "onError": 0,
            "onNull": 0
          }
        }
      }
    }
  }
])

{ "_id" : 0, "sum" : 281245 }