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

To flatten this array, you need to shift process to client.

mongo will provide some new flattening options in new edition, but afaik it will be arithmetic ones (avg, min, max....).