如果为空，如何使用 $subtract 和条件来使用数组中的默认值？

Question

我正在尝试从聚合管道中的 final 数组和 start 数组中减去值。但也有一些特殊情况需要在减法之前添加一些额外的逻辑。

预期输出：

1. I need to subtract nth value of startarray from nth value of final array
2. And then, get the total sum of subtracted values

特殊情况：

1. If nth value of start array is NULL, use a start_default value(from the query)
2. If nth value of final array is NULL, use value from the final_default array

经过一些聚合阶段后，我的 MongoDB 文档具有以下格式：

Assuming that start_default value = 1

I have commented the way I expect to perform subtractions in each of the group

{
  "data": [
    {
      "key": "TP-1",
      "status_map": [
        {
          "status": "Closed",
          "final": [
            6,
            3
          ],                               // Expected Output
          "start": [                       // sum:6 [(6-2)+(3-1(start_default))=4+2]
            2
          ],
          "final_default": [
            4
          ]
        },
        {
          "status": "Done",
          "final": [
            4
          ],                              // Expected Output
          "start": [                     // sum:2 [(4-3)+(2(final_default)-1)=1+1]
            3,
            1                            
          ],
          "final_default": [
            2
          ]
        }
      ]
    },
    {
      "key": "TP-2",
      "status_map": [
        {
          "status": "Closed",
          "final": [
            1,
            5
          ],                             // Expected Output
          "start": [],                   //sum:4 [(1-1(start_default))+(5-1(start_default))=0+4]
          "final_default": [
            3
          ]
        },
        {
          "status": "Done",
          "final": [],                    // Expected Output
          "start": [                     //sum:3 [(5(final_default)-3)+(5(final_default)-4)=2+1]
            3,
            4
          ],
          "final_default": [
            5
          ]
        }
      ]
    }
  ]
}

假设 start_default 值 = 1

，这是我的预期输出

{
  "data": [
    {
      "key": "TP-1",
      "status_map": [
        {
          "status": "Closed",
          "sum": 6  //[(6-2)+(3-1(start_default))=4+2]
        {
          "status": "Done",
          "sum": 2 //[(4-3)+(2(final_default)-1)=1+1]
        }
      ]
    },
    {
      "key": "TP-2",
      "status_map": [
        {
          "status": "Closed",
          "sum": 4  //[(1-1(start_default))+(5-1(start_default))=0+4]
        },
        {
          "status": "Done",
          "sum": 3 //[(5(final_default)-3)+(5(final_default)-4)=2+1]
        }
      ]
    }
  ]
}

如何实现这个用例？

Answer 1

您可以从双 $map to rewrite your nested array. You'll also need $reduce since you'll be converting an array into scalar value. Since you need to "pair" two arrays, there's a perfect operator called $zip 开始，即使数组长度不同也可以使用。为第一个子文档配对 final 和 start 将 return:

[ [ 6,2 ], [ 3, null ] ]

这很棒，因为您可以使用 $ifNull 提供默认值。

您的汇总如下所示：

db.collection.aggregate([
    {
        $project: {
            data: {
                $map: {
                    input: "$data",
                    as: "d",
                    in: {
                        key: "$$d.key",
                        status_map: {
                            $map: {
                                input: "$$d.status_map",
                                as: "sm",
                                in: {
                                    status: "$$sm.status",
                                    sum: {
                                        $reduce: {
                                            input: {
                                                $zip: {
                                                    inputs: [ "$$sm.final", "$$sm.start" ],
                                                    useLongestLength: true
                                                }
                                            },
                                            initialValue: 0,
                                            in: {
                                                $add: [
                                                    "$$value",
                                                    {
                                                        $subtract: [
                                                            { $ifNull: [ { $arrayElemAt: [ "$$this", 0 ] }, { $arrayElemAt: [ "$$sm.final_default" , 0] } ] },
                                                            { $ifNull: [ { $arrayElemAt: [ "$$this", 1 ] }, 1 ] }
                                                        ]
                                                    }
                                                ]
                                            }
                                        }
                                    }
                                }
                            }
                        }
                    }
                }
            }
        }
    }
])

Mongo Playground