将数组内容过滤到包含 $concatArrays 的查询

给定这个函数，我有一个正在查询的数据集。数据如下：

db.activity.insert(
    {
        "_id" : ObjectId("5908e64e3b03ca372dc945d5"),
        "startDate" : ISODate("2017-05-06T00:00:00Z"),
        "details" : [
            {
                "code" : "2",
                "_id" : ObjectId("5908ebf96ae5003a4471c9b2"),
                "walkDistance" : "03",
                "jogDistance" : "01",
                "runDistance" : "08",
                "sprintDistance" : "01"
            }
        ]
    }
)

db.activity.insert(
    {
        "_id" : ObjectId("58f79163bebac50d5b2ae760"),
        "startDate" : ISODate("2017-05-07T00:00:00Z"),
        "details" : [
            {
                "code" : "2",
                "_id" : ObjectId("58f7948fbebac50d5b2ae7f2"),
                "walkDistance" : "01",
                "jogDistance" : "02",
                "runDistance" : "09",
                "sprintDistance" : ""
            }
        ]
    }
)

使用这个函数，感谢 Neil Lunn，我能够得到我想要的输出：

db.activity.aggregate([
  { "$project": {
    "_id": 0,
    "unique": {
      "$filter": {
        "input": {
          "$setDifference": [
            { "$concatArrays": [ 
              "$details.walkDistance",
              "$details.jogDistance",
              "$details.runDistance",
              "$details.sprintDistance"
            ]},
            []
          ]
        },
        "cond": { "$ne": [ "$$this", "" ] }
      }
    }
  }},
  { "$unwind": "$unique" },
  { "$group": {
    "_id": null,
    "uniqueArray": { "$addToSet": "$unique" }  
  }}
])

但是，我无法在开头添加匹配语句。

db.activity.aggregate([
  {$match: {"startDate" : ISODate("2017-05-06T00:00:00Z"), "details.code": "2" },
  {$unwind: '$details'},
  {$match: {"startDate" : ISODate("2017-05-06T00:00:00Z"), "details.code": "2" },
  { "$project": {
    "_id": 0,
    "unique": {
      "$filter": {
        "input": {
          "$setDifference": [
            { "$concatArrays": [ 
              "$details.walkDistance",
              "$details.jogDistance",
              "$details.runDistance",
              "$details.sprintDistance"
            ]},
            []
          ]
        },
        "cond": { "$ne": [ "$$this", "" ] }
      }
    }
  }},
  { "$unwind": "$unique" },
  { "$group": {
    "_id": null,
    "uniqueArray": { "$addToSet": "$unique" }  
  }}
])

因为它给出了以下错误消息：

> $concatArrays only supports arrays, not string

我怎样才能修改这个查询以便$match可以添加声明吗？

Don't $unwind https://docs.mongodb.com/manual/reference/operator/aggregation/unwind/您要输入的数组$concatArrays https://docs.mongodb.com/manual/reference/operator/aggregation/concatArrays/。而是申请$filter https://docs.mongodb.com/manual/reference/operator/aggregation/filter/仅提取匹配值。如上所述，我们可以使用$setUnion https://docs.mongodb.com/manual/reference/operator/aggregation/setUnion/对于“唯一串联”：

db.activity.aggregate([
  { "$match": { "startDate" : ISODate("2017-05-06T00:00:00Z"), "details.code": "2" } },
  { "$project": {
    "_id": 0,
    "unique": {
      "$let": {
        "vars": {
          "filtered": {
            "$filter": {
              "input": "$details",
              "cond": { "$eq": [ "$$this.code", "2" ] }
            }  
          }
        },
        "in": {
          "$setDifference": [
            { "$setUnion": [ 
              "$$filtered.walkDistance",
              "$$filtered.jogDistance",
              "$$filtered.runDistance",
              "$$filtered.sprintDistance"
            ]},
            [""]
          ]
        } 
      }
    }
  }},
  { "$unwind": "$unique" },
  { "$group": {
    "_id": null,
    "uniqueArray": { "$addToSet": "$unique" }  
  }}
])

Using $let https://docs.mongodb.com/manual/reference/operator/aggregation/let/使语法更加清晰，因为您不需要指定多个$map https://docs.mongodb.com/manual/reference/operator/aggregation/map/ and $filter https://docs.mongodb.com/manual/reference/operator/aggregation/filter/语句“内联”作为源$setUnion https://docs.mongodb.com/manual/reference/operator/aggregation/setUnion/