Mogodb aggregation create output as {"key": "value"} from 2 arrays

Question

I have a question about how to create output from 2 arrays one array with translation key and another with translation i would output as

"translation_key":"value"  

Current output:
{

        "_id" : ObjectId("5bfc0b2b30c4683f585078fb"),
        "translation" : [ 
            "hour", 
            "day"

        ],
        "translation_key" : [ 
            "HOUR_TEXT", 
            "DAY_TEXT"  


        ],
        "locale_id" : "EN_en"
   }

OUTPUT should be :

{
    "EN_en" :{
        "HOUR_TEXT" :"hour",
        "DAY_TEXT" :"day",
    }
}

Answer 1

You can try below aggregation:

db.col.aggregate([
    {
        $project: {
            array: [
                {
                    k: "$locale_id",
                    v: {
                        $arrayToObject: {
                            $map: {
                                input: { $range: [0, { $size: "$translation" }] },
                                as: "index",
                                in: {
                                    k: { $arrayElemAt: [ "$translation", "$$index" ] },
                                    v: { $arrayElemAt: [ "$translation_key", "$$index" ] }
                                }
                            }
                        }
                    }
                }
            ]
        }
    },
    {
        $replaceRoot: {
            newRoot: { $arrayToObject: "$array" }
        }
    }
])

Basically you need to use $arrayToObject operator to manipulate your key names and in your case it should be used twice. This operator expects an array of objects with two properties k and v and therefore you should use $map to generate that values based on translation, $range is used to generate indexes to traverse translation and translation_key arrays. Then you can use $replaceRoot to promote dynamically generated key into root level of your document.

Output:

{ "EN_en" : { "hour" : "HOUR_TEXT", "day" : "DAY_TEXT" } }