How to return another field and distinct field from mongodb distinct

Question

Hello guys I want to ask, how can I return another field in the result of distinct mongodb method?

For example, I have the following documents:

[
    {
        "id": 1,
        "segment": "AAS",
        "partner": {}
    },
    {
        "id": 2,
        "segment": "AAS",
        "partner": {}
    },
    {
        "id": 3,
        "segment": "BBS",
        "partner": {}
    },
    {
        "id": 4,
        "segment": "CCS",
        "partner": {}
    },
    {
        "id": 5,
        "segment": "DDS",
        "partner": {}
    }
]

I want to distinct the segment field, but, I need to return the partner field too:

[
    {
        "segment": "AAS",
        "partner": {}
    },
    {
        "segment": "BBS",
        "partner": {}
    },
    {
        "segment": "CCS",
        "partner": {}
    },
    {
        "segment": "DDS",
        "partner": {}
    }
]

How can I produce that? Is it possible to use mapReduce method? how can I implement that?

Thank you!

Answer 1

Assume that you don't care about which AAS you get. I pick the first one.

db.collection.aggregate([
  {
    $group: {
      _id: "$segment",
      field: {
        $first: "$$ROOT"
      }
    }
  },
  {
    $replaceWith: "$field"
  },
  {
    $unset: [ "id", "_id" ]
  }
])

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