I'm looking for a way to search a Mongo object for a specific key, and return its value and path if found.

The questions I've seen related to this use dot notation and $exists, since they know the structure of the object. In this case, however, the target field can sometimes be nested three levels deep and sometimes just one level, and I do not know the path to it or its parent key.

CodePudding user response：

Check my mongoplayground below, it can be used up to three level of array. If you want more level, just copy some of code and you can make it.

data

[
  {
    _id: 1,
    "key": [
      {
        "key2": [
          {
            "key3": [
              {}
            ]
          }
        ]
      }
    ]
  },
  {
    _id: 2,
    "key4": 2
  }
]

aggregate

db.collection.aggregate([
  {
    $set: { n1: { $objectToArray: "$$ROOT" } }
  },
  {
    $unwind: {
      path: "$n1",
      preserveNullAndEmptyArrays: true
    }
  },
  {
    $unwind: {
      path: "$n1.v",
      preserveNullAndEmptyArrays: true
    }
  },
  {
    $set: {
      key_array: [ "$n1.k" ],
      n2: {
        $cond: {
          if: { $eq: [ { $type: "$n1.v" }, "object" ] },
          then: { $objectToArray: "$n1.v" },
          else: null
        }
      }
    }
  },
  {
    $unwind: {
      path: "$n2",
      preserveNullAndEmptyArrays: true
    }
  },
  {
    $unwind: {
      path: "$n2.v",
      preserveNullAndEmptyArrays: true
    }
  },
  {
    $set: {
      key_array: {
        $concatArrays: [ "$key_array", [ "$n2.k" ] ]
      },
      n3: {
        $cond: {
          if: { $eq: [ { $type: "$n2.v" }, "object" ] },
          then: { $objectToArray: "$n2.v" },
          else: null
        }
      }
    }
  },
  {
    $unwind: {
      path: "$n3",
      preserveNullAndEmptyArrays: true
    }
  },
  {
    $unwind: {
      path: "$n3.v",
      preserveNullAndEmptyArrays: true
    }
  },
  {
    $set: {
      key_array: {
        $concatArrays: [ "$key_array", [ "$n3.k" ] ]
      }
    }
  },
  {
    $group: {
      _id: "$_id",
      key_array: { $addToSet: "$key_array" }
    }
  },
  {
    $set: {
      key_array: {
        $reduce: {
          input: "$key_array",
          initialValue: [],
          in: { $concatArrays: [ "$$value", "$$this" ] }
        }
      }
    }
  },
  {
    $unwind: "$key_array"
  },
  {
    $match: { key_array: { $ne: null } }
  },
  {
    $group: {
      _id: "$_id",
      key_array: { $addToSet: "$key_array" }
    }
  }
])

result

[
  {
    "_id": 1,
    "key_array": [
      "key",
      "key3",
      "_id",
      "key2"
    ]
  },
  {
    "_id": 2,
    "key_array": [
      "_id",
      "key4"
    ]
  }
]

mongoplayground