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

Sometimes it's easiest to use JavaScript:

db.getCollection('Collection1').aggregate([
{
   $lookup:
     {
       from: 'Collection2',
       localField: 'id',
       foreignField: 'collection1_id',
       as: 'col2'
     }
}]).map((e) => ({
  id: e.id,
  field: e.field,
 collection2: Array.isArray(e.col2) &&
   e.col2.reduce((arr, el) => {
     arr.push(el.name);
    return arr;
  }, []).join(', ')
}))