MongoDb Aggregation - project values as keys and corresponding array value as values

Question

In mongodb, I have a get object in below shape after running aggregation.

 {
      "_id": 1,
      "specificationList": {
        "key": "Memory & Storage Features",
        "values": [
          {
            "key": "Internal Storage",
            "value": [
              "32 KB"
            ]
          },
          {
            "key": "RAM",
            "value": [
              "32 MB"
            ]
          },
          {
            "key": "Expandable Storage",
            "value": [
              "8 GB"
            ]
          },
          {
            "key": "Supported Memory Card Type",
            "value": [
              "MicroSD"
            ]
          }
        ]
      }
    }

From above document , how could I get object in below shape , in next aggregation pipeline. I need to get to below shape so as to make code cleaner. I am using aggregation to arrive at above shape (so want to append another pipeline), and it would be nice to know what aggregation pipeline to get at below shape

{
 "specList” : {
    “Internal Storage”: “32 KB”,
   “RAM”:”32 MB”,
   “Expandable Storage”:”8 GB”,
    “Supported Memory Card Type”:”MicroSD”
   }
}

Answer 1

Without knowing the full pipeline, you can use the $arrayToObject operator which converts an array into a single document but the array must contain two fields, k and v where the k field contains the field name and the v field contains the value of the field. In the above aggregate document you would need to map the values array to the above format which the $arrayToObject operator can then happily convert.

Consider adding a $project pipeline step which uses the $map operator to change the array structure and then apply the result of the transformation to the desired object.

The following illustrates this:

db.collection.aggregate([
    { ... }, // <-- previous pipeline
    {
        "$project": {
            "specList": {
                "$arrayToObject": {
                    "$map": {
                        "input": "$specificationList.values",
                        "as": "el",
                        "in": {
                            "k": "$$el.key",
                            "v": { "$arrayElemAt": ["$$el.value", 0] }
                        }
                    }
                }
            }
        }
    }
])