Get key of max value from unnamed nested array of objects in Mongo DB aggregate

Question

I have a document that looks like this.

{
  city: "Manhattan",
  State: "NY",
  eth:[
    {02: 40},
    {15:50},
    {04:10}
  ]

}

What I wanna do is find the object within the eth array with the max value. I have tried doing this.

project:{
  {'eth':{$reduce:{
                        input:"$eth",
                        initialValue:1,
                        in:{$max:["$$value","$$this"]}
                    }}}
            ,{
            $project:{"eth":{$objectToArray:"$eth"}},
        }
}

which results into [{k: 15, v:60}].

How do I get the value of k as my eth. I tried doing another project where I tried to get "$eth[0].k" but it results as undefined.

I want the output to look like :

{
  city: "Manhattan",
  state:"NY",
  eth: 15
}

Ashh · Accepted Answer · 2020-04-19 12:09:50Z

1

You can simply use $max to get the max value

db.collection.aggregate([
  { "$project": {
    "max": {
      "$objectToArray": {
        "$max": "$eth"
      }
    }
  }},
  { "$project": {
    "eth": {
      "$arrayElemAt": ["$max.k", 0]
    }
  }}
])

edited Apr 19, 2020 at 12:09

answered Apr 16, 2020 at 14:30

Ashh

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2 Comments

sajak upadhyaya Over a year ago

That works for max. Is there a way I could just get the key for the max value since it has no name. I need to group based on the key right after this project stage

sajak upadhyaya Over a year ago

Thank you, I did something similar but I just did eth.k to access in group statement.

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