Convert array of objects to the object, with keys based on object keys?

Question

For example, in my Mongo Aggregate request I'm getting the array, which consists of two objects:

[
  {
    "key": "first",
    "meta": 123,
  },
  {
    "key": "second",
    "meta": 567,
    "maybeonmorefield": 789
  }
]

Is it possible to convert this array to the following object by using only Mongo aggregation framework:

{
  "first": {
    "meta": 123,
  },
  "second": {
    "meta": 123,
    "maybeonmorefield": 789
  }
}

As you see, the keys are actually the key field values from the objects in the first examples.

Thanks for any tips :)

mickl · Accepted Answer · 2019-12-10 14:52:52Z

1

You can use $replaceRoot to re-arrange your documents on root level and $arrayToObject to build new keys dynamically:

db.collection.aggregate([
    {
        $addFields: {
            "root": "$$ROOT"
        }
    },
    {
        $project: {
            "root.key": 0,
            "root._id": 0
        }
    },
    {
        $replaceRoot: {
            newRoot: {
                $arrayToObject: [[ { k: "$key", v: "$root" } ]]
            }            
        }
    }
])

Mongo Playground

answered Dec 10, 2019 at 14:52

mickl

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