Question 115247 in Mongodb Question posted in
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Mongodb aggregation pipeline to get subdocument object as string

I have a MongoDB collection containing documents like these:

   {
    "id": "62cd5de762c595905e37821b",
    "letterNo": "122233",
    "letterDate": "2023-03-01",
    "metaData": {
      "name": "test",
      "family ": "test"
     }
  },
   {
    "id": "62cd5de762c595905e37821a",
    "letterNo": "122233",
    "letterDate": "2023-03-01",
    "metaData": {
      "type": "xxxx",
      "where": "test"
     }
  }

I want to $group in metaData field as metaData_string, that every shema change to string live this:

   {
    "id": "62cd5de762c595905e37821a",
    "letterNo": "122233",
    "letterDate": "1400-03-01",
    "metaData_string":"{n'name': 'test',n'family ': 'test'n}"
  },
  {
    "id": "62cd5de762c595905e37821a",
    "letterNo": "122233",
    "letterDate": "1400-03-01",
    "metaData_string":"{n'type': 'xxxxx',n'where': 'test'n}"
  }

Is it possible using the Aggregation Framework only? I haven’t seen reference to subdocuments in the aggregation framework documentation, so I think it’s not…

Thx you all.

Tags:

2

Answers


  1. 0

    You can use $function, if it’s a one-time operation, but you’ll need to enable server-side javascript. Like this:

    db.collection.aggregate([
  {
    "$addFields": {
      "metaData": {
        "$function": {
          "body": "function(obj) { return JSON.stringify(obj)}",
          "args": [
            "$$ROOT.metaData"
          ],
          "lang": "js"
        }
      }
    }
  }
])

    Playground link.

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  2. 0

    Regarding the above answer due to security vulnerability such as JS injection I recommend using $map, $reduce operator to get the string result and concat them together, below is the solution I come up with that also handles the nested list of objects:

     { $project: {
   "metaData": {
     $map: {
       input: {
         $objectToArray: "$metaData"  // $map only get the array to we are converting the object to array and input it in metaData
       },
       as: "metaData",
       in: {
         $cond: {
           if: {
             $isArray: "$$metaData.v"   // check the if value of our metadata is is array if yes check the the null value first   and use $reduce to deconstruct the array in single value
           },
           then: {
             "customer": {
               $reduce: {
                 input: {
                   $ifNull: [
                     "$$metaData.v",
                     []
                   ]
                 },
                 initialValue: "",
                 in: {
                   $map: {
                     input: {
                       $objectToArray: "$$this"  // as we have  inner array use another map to get the objects value  and use $toString to convert the values to string
                     },
                     as: "cdata",
                     in: {
                       $concat: [
                         {
                           $toString: "$$cdata.k"
                         },
                         "":"",
                         {
                           $toString: "$$cdata.v"
                         }
                       ]
                     }
                   }
                 }
               }
             },
             
           },
           else: {      // this part  returns the non-array object values
             $concat: [
               "{ "",
               "$$metaData.k",
               "": "",
               "$$metaData.v",
               "" }"
             ]
           }
         }
       }
     }
   }
 }
}

    Playground_Link

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