您可以：

1. $objectToArray，将$$ROOT文档转换为k-v对数组，以供将来查找
   1.使用$lookup中的子管道通过uuid查找值
   1.使用$mergeObject将原始值（即"20200206 ..."）与查找到的新字段名称（即"Phone ..."）组合在一起
   1.使用$arrayToObject和$replaceRoot将结果转换回原始形式

db.FormData.aggregate([
  {
    $match: {
      "_id": ObjectId("5e3c27bf1ef77236945ef07b")
    }
  },
  {
    $project: {
      kv: {
        "$objectToArray": "$$ROOT"
      }
    }
  },
  {
    $unwind: "$kv"
  },
  {
    "$lookup": {
      "from": "Form",
      "let": {
        uuid: "$kv.k"
      },
      "pipeline": [
        {
          $match: {
            "_id": ObjectId("5e3c27bf1ef77236945ef07b")
          }
        },
        {
          "$unwind": "$Fields"
        },
        {
          $match: {
            $expr: {
              $eq: [
                "$$uuid",
                "$Fields.FieldID"
              ]
            }
          }
        },
        {
          $project: {
            _id: false,
            k: "$Fields.Title"
          }
        }
      ],
      "as": "formLookup"
    }
  },
  {
    $unwind: "$formLookup"
  },
  {
    $project: {
      kv: {
        "$mergeObjects": [
          "$kv",
          "$formLookup"
        ]
      }
    }
  },
  {
    $group: {
      _id: "$_id",
      kv: {
        $push: "$kv"
      }
    }
  },
  {
    "$project": {
      newDoc: {
        "$arrayToObject": "$kv"
      }
    }
  },
  {
    "$replaceRoot": {
      "newRoot": {
        "$mergeObjects": [
          {
            "_id": "$_id"
          },
          "$newDoc"
        ]
      }
    }
  }
])

Mongo Playground

另一种选择是从Form集合开始并避开$unwind：

1. $match和$lookup，将所有需要的数据放入一个文档
2. $objectToArray以获取FormData的已知密钥
   1.使用$indexOfArray和$arrayElemAt匹配项，并使用$mergeObjects合并它们。然后使用arrayToObject化响应

db.Form.aggregate([
  {$match: {_id: ObjectId("5e3c27bf1ef77236945ef07b")}},
  {$lookup: {
      from: "FormData",
      localField: "_id",
      foreignField: "_id",
      as: "formLookup",
      pipeline: [{$project: {_id: 0}}]
  }},
  {$set: {formLookup: {$objectToArray: {$first: "$formLookup"}}}},
  {$replaceRoot: {
      newRoot: {
        $mergeObjects: [
          {$arrayToObject: {
              $map: {
                input: "$formLookup",
                in: {$mergeObjects: [
                    {v: "$$this.v"},
                    {k: {$getField: {
                          input: {$arrayElemAt: [
                                "$Fields", 
                                {$indexOfArray: ["$Fields.FieldID", "$$this.k"]}
                          ]},
                          field: "Title"
                    }}}
                ]}
              }
          }},
          {_id: "$_id"}
        ]
      }
  }}
])

了解它在playground example上的工作原理