Concatenate string values in array in a single field in MongoDB

Question

Suppose that I have a series of documents with the following format:

{
    \"_id\": \"3_0\",
    \"values\": [\"1\", \"2\"]
}

and I would l

Answer 1

You can use the reduce operator together with the substr operator.

db.collection.aggregate([
{
    $project: {
        values: {
            $reduce: {
              input: '$values',
              initialValue: '',
              in: {
                $concat: ['$$value', '_', '$$this']
              }
            }
        }   
    }       
},
{
    $project: {
        values: { $substr: ['$values', 1 , -1]}
    }       
}])