Find max element inside an array

Question

REF: MongoDB Document from array with field value max

Answers in Finding highest value from sub-arrays in documents and MongoDB find by max value in array of documents suggest to use sort + limit(1), however this is really slow. Surely there is a way to use the $max operator.

Suppose one gets a document like this in an aggregate match:

{
  _id: "notImportant",
  array: [
    {
      name: "Peter",
      age: 17
    },
    {
      name: "Carl",
      age: 21
    },
    {
      name: "Ben",
      age: 15
    }
  ]
}

And you want to find the (entire, not just the one value) document where age is highest. How do you do that with the $max operator?

I tried

unwind {"$array"}
project {"_id": 0, "name": "$array.name", "age": "$array.age"}

so I get

{
  _id: null,
  name: "Peter",
  age: 17
}
{
  _id: null,
  name: "Carl",
  age: 21
}
{
  _id: null,
  name: "Ben",
  age: 15
}

Then I tried matching age:

age: {$eq: {$max: "$age"}}

, but that gives me no results.

In other words what I need to get is the name and all other fields that belong to the oldest person in the array. And there are many thousands of persons, with lots of attributes, and on top of it all it runs on a raspberry pi. And I need to do this operation on a few dozen of such arrays. So with the sorting this takes about 40 seconds all in all. So I would really like to not use sort.

Answer 1

can you try this aggregation with $reduce

db.t63.aggregate([
    {$addFields : {array : {$reduce : {
        input : "$array", 
        initialValue : {age : 0}, 
        in : {$cond: [{$gte : ["$$this.age", "$$value.age"]},"$$this", "$$value"]}}
    }}}
])

output

{ "_id" : "notImportant", "array" : { "name" : "Carl", "age" : 21 } }