Question 174610 in Mongodb Question posted in
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Mongodb – How to convert array of subdocuments into array of primary data values returned from a $lookup?

The $lookup stage will return an array of subdocuments. But the only value that I am interested in from the result of the $lookup is the the _id value. So instead of an array of { _id: ObjectId() }, how do I flatten this array into just an array of ObjectId: [ ObjectId(), Objectid(), ... ]

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2

Answers


  1. 0 
    db={
  "orders": [
    {
      key: 1,
      
    },
    {
      key: 2,
      
    },
    {
      key: 3,
      
    }
  ],
  "inventory": [
    {
      id: 1,
      name: "a"
    },
    {
      id: 2,
      name: "b"
    },
    {
      id: 3,
      name: "c"
    },
    {
      id: 4,
      name: "d"
    }
  ]
}

    Aggregation :

    db.orders.aggregate([
  {
    "$lookup": {
      "from": "inventory",
      "localField": "key",
      "foreignField": "id",
      "as": "result"
    }
  },
  {
    "$project": {
      _id: 0,
      result: 1
    }
  },
  {
    "$unwind": "$result"
  },
  {
    "$group": {
      "_id": null,
      "result": {
        "$push": "$result.id"
      }
    }
  }
])

    Output : returns ID in array (we can have mongodb objectId) but i have used simple numeric ID

    [
  {
    "_id": null,
    "result": [
      1,
      2,
      3
    ]
  }
]
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  2. 0

    You can simply map the array of subdocuments, using $map, like this:

    db.orders.aggregate([
  {
    "$lookup": {
      "from": "inventory",
      "localField": "key",
      "foreignField": "id",
      "as": "result"
    }
  },
  {
    "$addFields": {
      "result": {
        "$map": {
          "input": "$result",
          "as": "item",
          "in": "$$item._id"
        }
      }
    }
  }
])

    Playground link

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