Mongodb，带条件的$sum

Question

文件:

[
  {
    name: 'abc'
    length: 25,
    area: 10
  },
  {
    name: 'abc',
    length: 5
  }
]

聚合查询后的输出:

[
  {
     count: 2,
     summarizedLength: 30,
     summarizedArea: null,
     _id: {
       name: 'abc'
     }
  }
]

长度和面积应该总结。但前提是所有文档都具有 area 或 length 属性。

因此，如果分组属性缺少任何 length 属性，则 summarizedLength 值应为 null/undefined/not exisitng，并且相同用区域。

我试过这个:

let query = mongoose.model('mycollection').aggregate([
    {
      $group: {
        _id: {
          name: $name
        },
        count: {
          $sum: 1
        },
        summarizedLength: { $sum: "$length" },
        summarizedArea: { $sum: "$area" },
      }
    }
  ]);

问题是，如果缺少任何属性，我需要取消 $sum。这可能吗？

Answer 1

来自 Mongo 文档 $sum behavior

If used on a field that contains both numeric and non-numeric values, $sum ignores the non-numeric values and returns the sum of the numeric values.

If used on a field that does not exist in any document in the collection, $sum returns 0 for that field.

If all operands are non-numeric, $sum returns 0.

我们可以$push所有的面积和长度到数组，然后比较count和数组的长度

db.n.聚合(

    [ 
        {
            $group: {
                _id: { name: "$name" }, 
                count: { $sum: 1 }, 
                area : {$push : "$area"}, 
                length : {$push : "$length"} } 
        },
        {
            $project:{
                _id: "$_id",
                count: "$count",
                summarizedLength: { $cond : [ {$eq : [ "$count", {$size : "$length"} ]} , { $sum : ["$length"] }, "not all numbers" ] },
                summarizedArea: { $cond : [ {$eq : [ "$count", {$size : "$area"} ]} , { $sum : ["$area"] }, "not all numbers" ] },
            }
        }
    ] 
)

或者，我们可以计算定义的长度和面积的数量，以及总的计数，如果计数匹配，那么所有数字都是未定义的。

要严格检查类型，以防 area 和 length 可能包含非数字数据，而不是 undefined 我们可以做 $type 检查

db.n.aggregate(
    [
        {
            $group: {
                _id: { name: "$name" },
                count: { $sum: 1 },
                areaCount : { $sum : { $cond : [ {$eq : [ "$area", undefined ]} , 0, 1 ] } },
                lengthCount : { $sum : { $cond : [ {$eq : [ "$length", undefined ]} , 0, 1 ] } },
                summarizedLength: { $sum: "$length"  },
                summarizedArea: { $sum: "$area"  }
            }
        },
        {
            $project : {
                _id : "$_id",
                count: "$count",
                summarizedLength: { $cond : [ {$eq : [ "$count", "$lengthCount" ]} , "$summarizedLength", "not all numbers" ] },
                summarizedArea: { $cond : [ {$eq : [ "$count", "$areaCount" ]} , "$summarizedArea", "not all numbers" ] },
            }
        }
    ]
).pretty()

输出

{
    "_id" : {
        "name" : "abc"
    },
    "count" : 2,
    "summarizedLength" : 30,
    "summarizedArea" : "not all numbers"
}