MongoDB聚合从嵌套数组中获取最后一个元素的字段

Question

我只想聚合和显示嵌套数组中的最后一个对象
这是我的数据库:

[{
  "firstName": "Shaun",
  "salary": [
    {
    "id":1,
    "rate": 250,
    },
    {
    "id":2,
     "rate": 290,
    }
  ]
},{
  "firstName": "Julian",
  "salary": [
    {
    "id":1,
     "rate": 750,      
    },
    {
    "id":2,
     "rate": 760,      
    },
    {
    "id":3,
     "rate": 790,      
    },
  ]
}
}]

我想要的结果是:

{"firstName": "Shaun", "rate":290},{"firstName": "Julian", "rate":790}

Answer 1

尝试使用以下聚合查询 $arrayElemAt从数组中获取最后一个元素 salary.rate :

db.collection.aggregate({
  $project: {
    firstName: 1,
    rate: { $arrayElemAt: [ "$salary.rate", -1 ] }
  }
})

测试: mongoplayground