Get all distinct keys of a nested object

I have following data in my collection:

[
    {
        _id: "2313123123",
        metadata: {
            path: "...",
            value: "...",
            name: "..."
        }
    },
    {
        _id: "2313123123",
        metadata: {
            path: "...",
            name: "...",
            origin: "...",
        }
    },
    {
        _id: "2313123123",
        metadata: {
            path: "...",
            source: "..."
        }
    },
]

I want to retrieve all distinct key names of the field metadata from my documents.
I want to retrieve ["path", "value", "name", "origin", "source"].

How can I query for this? Is this possible with the distinct method or do I need to use aggregate?

>Solution :

You’ll have to use an aggregate for this, sadly due to the nature of your needs this is going to be a very "expensive" pipeline to execute. There is no way to avoid iterating over the entire collection and adding the unique keys to the array.

We’re going to use $objectToArray to turn metadata into an array, then $unwind it and finally using $group we could save all the unique values.

db.collection.aggregate([
  {
    $project: {
      keys: {
        $map: {
          input: {
            "$objectToArray": "$metadata"
          },
          in: "$$this.k"
        }
      }
    }
  },
  {
    $unwind: "$keys"
  },
  {
    $group: {
      _id: null,
      keys: {
        "$addToSet": "$keys"
      }
    }
  }
])

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