Add field of array element in MongoDB aggregation

Question

I have a collection of documents containing an array of objects:

db.collection.insert({
    arr: [
      { id: 1, text         


        

Answer 1

You can use $let with $arrayElemAt to define temporary variable and then reference it to get text field:

db.collection.aggregate([
    {
        $addFields: {
            text1: {
                $let: {
                    vars: {
                        first: {
                            $arrayElemAt: [ "$arr", 0 ]
                        }
                    },
                    in: "$$first.text"
                }
            }
        }
    }
])