After applying $filter in mongo, project specific nested array attributes

Question

The collection I'm trying to query has documents with the following structure -

{
  "_id": 1,
  "topA": "topAValue",
  "topB": "topBValue",
  "topC": "topCValue",
  "nestedDocArray": [
    {
      "attr1": "a",
      "attr2": "b",
      "attr3": "c"
    },
    {
      "attr1": "a5",
      "attr2": "b5",
      "attr3": "c5"
    },
    {
      "attr1": "a1000",
      "attr2": "b1000",
      "attr3": "c1000"
    }
  ]
}

I'm trying to query this document with "_id": 1 with a requirement to project only certain attributes. In addition to this, the requirement is to only fetch nestedDocArray which matches the condition "attr1": "a5".

The query I tried is as below -

db.testCollection.aggregate(
[
  {
    "$match": {
      "_id": 1
    }
  },
  {
    "$project": {
      "topA": 1,
      "nestedDocArray": {
        "$filter": {
          "input": "$nestedDocArray",
          "as": "nestedDocArray",
          "cond": {
            "$eq": [
              "$$nestedDocArray.attr1",
              "a5"
            ]
          }
        }
      }
    }
  }
]
);

The response of this query looks something like below -

{
  "_id": 1,
  "topA": "topAValue",
  "nestedDocArray": [
    {
      "attr1": "a5",
      "attr2": "b5",
      "attr3": "c5"
    }
  ]
}

This is fine. This has managed to project attributes topA and nestedDocArray. I further want to only project nestedDocArray.attr2. The output i'm looking for is like below.

{
  "_id": 1,
  "topA": "topAValue",
  "nestedDocArray": [
    {
      "attr2": "b5"
    }
  ]
}

How can I modify the query to achieve the same?

Possible dupe of stackoverflow.com/q/44914367/2683814

s7vr
– s7vr

2018-08-07 18:55:05 +00:00
Commented Aug 7, 2018 at 18:55 — s7vr
– s7vr, Commented Aug 7, 2018 at 18:55

mickl · Accepted Answer · 2018-08-07 18:56:02Z

1

You can use $map with $filter to reshape your data:

db.testCollection.aggregate([
    {
        $match: { _id: 1 }
    },
    {
        $project: {
            topA: 1,
            nestedDocArray: {
                $map: {
                    input: {
                        $filter: {
                            input: "$nestedDocArray",
                            as: "nestedDocArray",
                            cond: {
                                $eq: [ "$$nestedDocArray.attr1", "a5" ]
                            }
                        }
                    },
                    as: "item",
                    in: {
                        attr2: "$$item.attr2"
                    }
                }
            }
        }
    }
])

answered Aug 7, 2018 at 18:56

mickl

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