MongoDB Aggregate - How to get array of fields

Question

I am using MongoDB aggregation framework, suppose I am having collection structure like this:

[
  {
    _id: ObjectId(123)
    name: john,
    sessionDuration: 29
  },
  {
    _id: ObjectId(456)
    name: moore,
    sessionDuration: 45
  },
  {
    _id: ObjectId(789)
    name: cary,
    sessionDuration: 25
  },
]

I want to query and create a pipeline such that it returns something like this:

{
  durationsArr: [29, 49, 25, '$sessionDuration_Field_From_Document' ];
}

I am doing this because I want to get the average of durations from all the documents, so first add all of it into an array, then I will add the last stage where I do the $avg operation.

Any idea of how can I get the array of sessionDurationField? Or do you have any other best approach to calculate the sessionDuration average from the collection? Please thoroughly explain am new to MongoDB aggregation.

Yong Shun · Accepted Answer · 2023-01-05 08:16:52Z

1

$group - Group all documents.

1.1. $avg - Calculate the average of sessionDuration for all documents.

db.collection.aggregate([
  {
    $group: {
      _id: null,
      avgSessionDuration: {
        $avg: "$sessionDuration"
      }
    }
  }
])

Demo @ Mongo Playground

answered Jan 5, 2023 at 8:16

Yong Shun

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