MongoDB聚合。获取价值差异

我一直在与mongo进行斗争，试图找到一个解决方案来显示价值观之间的差异

我有这样的价值观：

[
    {val: 1},
    {val: 4},
    {val: 7},
    {val: 8},
    {val: 11}
]

[
    {diff: 3},
    {diff: 3},
    {diff: 1},
    {diff: 3}
]

我想收到这样的东西：

[
    {val: 1},
    {val: 4},
    {val: 7},
    {val: 8},
    {val: 11}
]

[
    {diff: 3},
    {diff: 3},
    {diff: 1},
    {diff: 3}
]

通过取下一个值（4）并减去上一个值（1），计算每个值。在所有这些之后，我们在输出中收到3，它作为第一项位于第二个列表中

---

是否可以使用MongoDB聚合实现它？

您需要将它们分组到阵列中，计算差异并再次展平

伪码 将以下步骤添加到管道中：

db.collection.aggregate([
  {
    $group: {
      _id: null,
      data: { $push: "$$ROOT" }
    }
  },
  {
    $addFields: {
      data: {
        $map: {
          input: {
            $range: [
              0,
              {
                $subtract: [
                  { $size: "$data" },
                  { $mod: [ { $size: "$data" }, 2 ] }
                ]
              },
              1
            ]
          },
          as: "idx",
          in: {
            diff: {
              $subtract: [
                {
                  $arrayElemAt: [
                    "$data.val",
                    {
                      $add: [ "$$idx", 1 ]
                    }
                  ]
                },
                {
                  $arrayElemAt: [ "$data.val", "$$idx" ]
                }
              ]
            }
          }
        }
      }
    }
  },
  {
    $unwind: "$data"
  },
  {
    $replaceRoot: {
      newRoot: "$data"
    }
  }
])

---

{val:1}

是单个文档还是数组项？它是聚合管道输出。（就像一份文件一样）