CODEWITHSUNDEEP
mongodbaggregation-framework
user10762393
user10762393

Reputation:

How do I move object values to an array in MongoDB?

I'm would like to do a data transformation using MongoDB Aggregation Framework. I'm stuck for several hours and can't seem to get my head around it.

Sample input data:

{
  a1: "value a1",
  a2: "value a2"
},
{
  a1: "value a3",
  a2: "value a4"
},
{
  a1: "value a5",
  a2: "value a6"
}

I would like to have the output below.

{
  a1: ["value a1", "value a3", "value a5"],
  a2: ["value a2", "value a4", "value a6"]
}

Upvotes: 1

Views: 1056

Answers (1)

mickl
mickl

Reputation: 49945

I'm assuming you want to run such aggregation dynamically (not knowing key names). In such case you need to start with $objectToArray, then run double $group to get an array for each key and in the last step you need to run $arrayToObject to get back key names as keys:

db.collection.aggregate([
    {
        $project: {
            doc: {
                $filter: {
                    input: { $objectToArray: "$$ROOT" },
                    cond: { $ne: [ "$$this.k", "_id" ] }
                }
            }
        }
    },
    {
        $unwind: "$doc"
    },
    {
        $group: {
            _id: "$doc.k",
            values: { $push: "$doc.v" }
        }
    },
    {
        $group: {
            _id: null,
            data: { $push: { k: "$_id", v: "$values" } }
        }
    },
    {
        $replaceRoot: {
            newRoot: {
                $arrayToObject: "$data"
            }
        }
    }
])

Mongo Playground

EDIT: the problem becomes very simple if you know the names of the keys:

db.collection.aggregate([
    {
        $group: {
            _id: null,
            a1: { $push: "$a1" },
            a2: { $push: "$a2" }     
        }
    }
])

Upvotes: 2

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