将数组的最后一个元素的字段与父文档的字段进行比较

Question

我必须将父文档的嵌入字段与最后一个文档的嵌入字段（存在于数组中）进行比较，后者也是父文档的一部分，并在这两个字段不匹配时返回文档的externalIds。

以下图片供参考：
[包含数组的实际元数据][1]
[这是包含该字段的另一张图片][2]

所以在第二张图片中，我实际上只想要最后一个修订，即修订数组中的最后一个文档与父文档进行比较，需要比较的字段是metadata.formData.mergeStatus.status。
我想要那些文档_ids，其中数组中的最后一个文档的字段与其父文档的相应字段不同，如果存在修订数组的话。

我是 mongodb 的新手，并且是第一次在 stackoverflow 中询问，所以如果你觉得它很愚蠢，请原谅，但是这是我的尝试。

db.document.aggregate([
    { $match: { $and: [
        { spaceId: 3007 },
        { revisions: { $exists: true, $not: { $size: 0 } } }
    ] } },
    { $project:
        { _id: 1,
        spaceId:1,
        externalId:1,
        revisions: { $cond: {
            if: { $isArray: "$revisions" },
            then: { $size: "$revisions" },
            else: "NA" } },
        last: { $arrayElemAt: [ "$revisions", -1 ] },
        difference: { $ne: [
            "last.metadata.formData.mergeStatus.status", 
            "metadata.formData.mergeStatus.status" ] }
        }
    }
]); 

The above suggestion really helps however i am confused for below scenario,
so the metadata field contains elementsData and inside of that is another embedded object signature, which has two string fields 
1.userSignatureDate(String)
2.userSignatureText(String)
so when i have applied above logic to compare the *userSignatureDate* of parent metadata with respective field in last document of  the revisions array.it is not giving actual results.
here is the image for the code that i had written and the sample metadata Structure .[enter image description here][3][enter image description here][4]


  [1]: https://i.stack.imgur.com/rsrKv.png
  [2]: https://i.stack.imgur.com/y6wsp.png
  [3]: https://i.stack.imgur.com/398I8.png
  [4]: https://i.stack.imgur.com/PYUAL.png

Answer 1

尝试类似的东西

db.getCollection('test').aggregate([
{$project:
      {
          status: "$metadata.formData.mergeStatus.status", 
          last_revisions: { $arrayElemAt: [ "$revisions", -1 ] }
      }},
{$project:
    {
        matched:{
            $eq:[
            "$status",
            "$last_revisions.metadata.formData.mergeStatus.status"
            ]}
       }}
])