Mongodb 在嵌套数组中的特定元素上投影

我有一份这样的文件

{
    "_id" : ObjectId("59b0ea9b3a91af574a3e0464"),
    "machineID" : "b83c",
    "sensorState" : [ 
        {
            "data" : "377",
            "sensor" : "solar",
            "time" : ISODate("2017-09-20T19:42:58.766Z")
        }, 
        {
            "data" : "35",
            "sensor" : "photosynthetic",
            "time" : ISODate("2017-09-20T19:42:58.782Z")
        }, 
        {
            "data" : "370",
            "sensor" : "solar",
            "time" : ISODate("2017-09-20T19:43:29.089Z")
        }, 
        {
            "data" : "400",
            "sensor" : "solar",
            "time" : ISODate("2017-09-20T19:44:29.089Z")
        }, 
        {
            "data" : "35",
            "sensor" : "photosynthetic",
            "time" : ISODate("2017-09-20T19:43:29.110Z")
        }
    ]
}

我只想检索

sensorState

数组中与

sensor=solar

匹配的子文档。所以，我试着这样做：

db.getCollection('sensorDB').find({ },
    { "sensorState" : { $elemMatch: {data : "35" } } })

但它只显示了一个结果：

{
    "_id" : ObjectId("59b0ea9b3a91af574a3e0464"),
    "sensorState" : [ 
        {
            "data" : "35",
            "sensor" : "photosynthetic",
            "time" : ISODate("2017-09-20T19:42:58.782Z")
        }
    ]
}

---

但是，我想在

sensorState

数组中查找与

sensor=solar

匹配的所有子文档。看起来，

$elemMatch

只能选择一个结果。

您正在尝试使用elemMatch进行项目，但根据

$elemMatch

运算符将查询结果中字段的内容限制为仅包含与

$elemMatch

条件匹配的第一个元素

因此，您看到的行为是elemMatch-in-a-projection的预期行为

您可以在聚合管道中使用

$project

和

$filter

来获得最终结果

给定OP中包含的示例文档，请使用以下命令

db.getCollection('elemMatch').aggregate([
  // projects on the sensorState sub documents and filters the output to only return sub 
  // documents having data=35
  {$project: {
      sensorState: {
        $filter: {
            input: "$sensorState",
            as: "sensorState",
            cond: { $eq: [ "$$sensorState.data", '35' ] }
          }
        }
      }
  }
])

。。。将返回：

{
    "_id" : ObjectId("59b0ea9b3a91af574a3e0464"),
    "sensorState" : [ 
        {
            "data" : "35",
            "sensor" : "photosynthetic",
            "time" : ISODate("2017-09-20T19:42:58.782Z")
        }, 
        {
            "data" : "35",
            "sensor" : "photosynthetic",
            "time" : ISODate("2017-09-20T19:43:29.110Z")
        }
    ]
}

您的问题显示试图在

data=35

上进行匹配，但也说明：

我要查找所有sensorState.sensor=solar

这是上面的命令，条件是：

db.getCollection('elemMatch').aggregate([
  // projects on the sensorState sub documents and filters the output to only return sub 
  // documents having sensor=solar
  {$project: {
      sensorState: {
        $filter: {
            input: "$sensorState",
            as: "sensorState",
            cond: { $eq: [ "$$sensorState.sensor", 'solar' ] }
          }
        }
      }
  }
])

返回：

{
    "_id" : ObjectId("59b0ea9b3a91af574a3e0464"),
    "sensorState" : [ 
        {
            "data" : "377",
            "sensor" : "solar",
            "time" : ISODate("2017-09-20T19:42:58.766Z")
        }, 
        {
            "data" : "370",
            "sensor" : "solar",
            "time" : ISODate("2017-09-20T19:43:29.089Z")
        }, 
        {
            "data" : "400",
            "sensor" : "solar",
            "time" : ISODate("2017-09-20T19:44:29.089Z")
        }
    ]
}

---

我自己找到了答案。使用聚合 答案是这样的

db.getCollection('sensorDB').aggregate(
        {$match: {machineID:"b83c"}},
        {$unwind:"$sensorState"},
        {$match: {"sensorState.sensor":"solar"}}
)

getCollection（'sensorDB'）.find（{'sensorState.data'：“35”，'sensorState.sensor'：“solar”}）。试试这个