嵌入式数组中的嵌入式文档中的项目字段 [英] Project field in embedded document within embedded array

问题描述

我有以下查询:

 cursor=self.postCol.aggregate([
      { "$graphLookup" : {
        "from": "pCol",
        "startWith": "$parent",
        "connectFromField": "parent",
        "connectToField": "_id",
        "as" : "parents"
        }
      },
      { "$project" : {
         "pValue": {
             "$cond": [
               { "$ne": [ "$pValue", [] ] },
               "$pValue",
               "$parents.0.pValue"
             ]
           }
         }
      }
    ])

先给出以下记录:

{"_id": 4, parent: "", "pValue": ["d"], fieldA: 9},
{"_id": 5, parent: 4, "pValue": [], fieldA:2},
{"_id": 6, parent: 4,"pValue": [], fieldA: 9}

我应该得到:

{"_id": 4, "pValue": ["d"]}
{"_id": 5, "pValue": ["d"]}
{"_id": 6, "pValue": ["d"]}

相反，我得到了:

{"_id": 4, "pValue": ["d"]}
{"_id": 5, "pValue": []}
{"_id": 6, "pValue": []}

由于某些我不理解的原因，$parents.0.pValue没有返回正确的值.如何在Parents数组的第一个元素中选择pValue的字段?

The $parents.0.pValue isn't returning the right value for some reason which I don't understand. How can I select the field of pValue within the first element of the parents array?

我想使用{"$arrayElemAt": ["$parents",0]}它将为我提供嵌入式文档，但是然后如何从中获取要返回的字段?

I thought to use {"$arrayElemAt": ["$parents",0]} which will give me the embedded document but then how can I then get a field from that to be returned?

推荐答案

您可以使用$let运算符在下面替换else条件.

You can replace the else condition with below using $let operator.

 {
    $let: {
        vars: {
            obj: {
                "$arrayElemAt": ["$parents", 0]
            }
        },
        in: "$$obj.pValue"
    }
 }

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