MongoDB aggregate value of key at array where index is given

Question

get value from items.z where y=five

Sample data

 [
 {
 "_id": 1,
 "items": [
   {
     x: "one",
     y: "two",
     z: "three"
   },
   {
     x: "four",
     y: "five",
     z: "six"
   }
 ]
 }]

Expected result

 [{
 "_id": 1,
 "indexValue": "six"
  }]

Tried query

db.collection.aggregate([
{
 $match: {
   _id: 1
 }
},
{
 $project: {
   indexValue: {
     $arrayElemAt: [
       "$items",
       1
     ]
   }
 }
},
{
 $project: {
   indexValue: "$indexValue.z"
 }
}
 ])

Here i dont know index and it take 2 pipelines

So how to get that value and can do it in one pipeline because of performance issue?

nimrod serok · Accepted Answer · 2022-08-10 14:40:40Z

2

One option is to use $filter:

db.collection.aggregate([
  {$match: {_id: 1}},
  {$project: {
      item: {
        $first: {
          $filter: {
            input: "$items",
            cond: {$eq: ["$$this.y", "five"]}
          }
        }
      }
    }
  },
  {$project: {indexValue: "$item.z"}}
])

See how it works on the playground example

answered Aug 10, 2022 at 14:40

nimrod serok

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4 Comments

Jalpen Bhatt Over a year ago

For more curiosity , Can i reduce last pipeline?

nimrod serok Over a year ago

What do you mean?

bhavesh Over a year ago

i think he wants to eliminate(remove) {$project: {indexValue: "$item.z"}} this pipeline

nimrod serok Over a year ago

Sure you can, but it will not give you the result you asked for. The playground example in the answer, allows to look at the pipeline results step-by-step, to better understand the pipeline. Look for the drop-down with the steps list

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