Question

我在MongoDB中有一个数据集，这是我的数据行的一个例子：

{ "conversionDate": "2016-08-01",
  "timeLagInDaysHistogram": 0,
  "pathLengthInInteractionsHistogram": 4,
  "campaignPath": [ 
      {"campaignName": "name1", "source": "sr1", "medium": "md1", "click": "0"},
      {"campaignName": "name2", "source": "sr1", "medium": "md1", "click": "0"},
      {"campaignName": "name1", "source": "sr2", "medium": "md2", "click": "1"},
      {"campaignName": "name3", "source": "sr1", "medium": "md3", "click": "1"} 
  ],
  "totalTransactions": 1,
  "totalValue": 37.0,
  "avgCartValue": 37.0
}

（campaignPath的长度不是常数，因此每行可以有不同数量的元素。

我想找到符合＆＃34; source = sr1＆＃34;的元素。 在campaignPath的最后一个元素。

我知道我不能用

这样的查询

db.paths.find(
    {
        'campaignPath.-1.source': "sr1"
    }
)

但是，因为我有＆＃34; pathLengthInInteractionsHistogram＆＃34;存储的等于广告系列路径长度的长度，我不会做类似的事情：

db.paths.find(
    {
        'campaignPath.$pathLengthInInteractionsHistogram.source': "sr1"
    }
)

Answer 1

从MongoDB 3.2开始，您可以使用aggregate执行此操作，-1提供$arrayElemAt运算符，该运算符接受db.paths.aggregate([ // Project the original doc along with the last campaignPath element {$project: { doc: '$$ROOT', lastCampaign: {$arrayElemAt: ['$campaignPath', -1]} }}, // Filter on the last campaign's source {$match: {'lastCampaign.source': 'sr1'}}, // Remove the added lastCampaign field {$project: {doc: 1}} ])索引来访问最后一个元素。

$where

在早期版本中，您使用db.paths.find({ $where: 'this.campaignPath[this.pathLengthInInteractionsHistogram-1].source === "sr1"' })卡住了。这会有效，但表现不佳：

pathLengthInInteractionsHistogram

您也可以在不使用db.paths.find({$where: 'this.campaignPath[this.campaignPath.length-1].source === "sr1"'})的情况下执行此操作：

let e = StaticEnum() // error: 'StaticEnum' cannot be constructed because it has no accessible initializers
let s = StaticStruct() // Useless, but legal

当数组大小存储在变量中时，查询MongoDB中数组的最后一个元素

1 个答案: