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

Starting in Mongo 4.4, the $function aggregation operator allows applying a custom javascript function to implement behaviour not supported by the MongoDB Query Language.

For instance, in order to concatenate an array of strings:

// { "_id" : "3_0", "values" : [ "1", "2" ] }
db.collection.aggregate(
  { $set:
    { "values":
      { $function: {
          body: function(values) { return values.join('_'); },
          args: ["$values"],
          lang: "js"
      }}
    }
  }
)
// { "_id" : "3_0", "values" : "1_2" }

$function takes 3 parameters:

• body, which is the function to apply, whose parameter is the array to join.
• args, which contains the fields from the record that the body function takes as parameter. In our case "$values".
• lang, which is the language in which the body function is written. Only js is currently available.