Mongo: Return specific fields from an array slice

Question

I would like to return specific fields from an array in Mongo and am having trouble.

Let us say we have a document like such:

{
  "student":"Bob",
  "report_cards": [
    {
      "Year":2016,
      "English":"B",
      "Math":"A"
    },
    {
      "Year":2015,
      "English":"B",
      "Math":"A"
    }
  ]
}

I would like to return the following:

{"Student": "Bob", {"English":"B"}}

Basically, I only need the first element in the report cards array, and only return the English field.

I know it is something around:

db.collection.find({},{"Student":1, "report_cards":{$slice:1}});

But this, of course, results in the full array (year, english, math) returning. I have tried the following:

db.collection.find({},{"Student":1, "report_cards.$.english":{$slice:1}})
db.collection.find({},{"Student":1, "report_cards":{$slice:1, "$.english"}});

but those aren't correct.

How do I simply return one field from the obj within the array results?

Thank you!

Mithun · Accepted Answer · 2017-08-03 17:30:28Z

3

You can use the aggregation framework to get the values as

db.test.aggregate
([{$project:{_id:0,
student:'$student',
English:{ $arrayElemAt: ['$report_cards.English',0]
}}}])

answered Aug 3, 2017 at 17:30

Mithun

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Comments

s7vr · Accepted Answer · 2017-08-03 17:44:10Z

AFAIK there is no built-in operator for filtering key value pair in embedded documents.

You can use combination of operators to achieve in latest 3.4 version.

The below query use $arrayElemAt to get first element in report_cards followed by $objectToArray to convert to array-view ( array of k and v pairs ) and $filter on English key to keep the matching key value pair and $arrayToObject to convert back to document.

Something like

{
  "$project": {
    "output": {
      "$arrayToObject": {
        "input": {
          "$filter": {
            "input": {
              "$objectToArray": {
                "$arrayElemAt": [
                  "$report_cards",
                  0
                ]
              }
            },
            "as": "result",
            "cond": {
              "$eq": [
                "$$result.k",
                "English"
              ]
            }
          }
        }
      }
    }
  }
}

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