MongoDB Aggregation join array of strings to single string

Question

We\'re trying to \'join\' an array of strings to a single string within an aggregation.

Given is the following dataset:

Collection 1:

{
  id: 123         


        

Answer 1

Starting Mongo 4.4, the $group stage has a new aggregation operator $accumulator allowing custom accumulations of documents as they get grouped:

// { "collectionId" : 1234, "name" : "Max"  }
// { "collectionId" : 876,  "name" : "Rob"  }
// { "collectionId" : 1234, "name" : "Andy" }
db.collection.aggregate([
  { $group: {
    _id: "$collectionId",
    names: {
      $accumulator: {
        accumulateArgs: ["$name"],
        init: function() { return [] },
        accumulate: function(names, name) { return names.concat(name) },
        merge: function(names1, names2) { return names1.concat(names2) },
        finalize: function(names) { return names.join(",") },
        lang: "js"
      }
    }
  }}
])
// { "_id" : 876,  "names" : "Rob"      }
// { "_id" : 1234, "names" : "Max,Andy" }

The accumulator:

• accumulates on the field name (accumulateArgs)
• is initialised to an empty array (init)
• accumulates by concatenating new names to already seen names (accumulate and merge)
• and finally joins all names as a string (finalize)