Call function inside mongodb's aggregate?

Question

Collection:

[
    { _id: \"Foo\", flag1: false, flag2: true, flag3: false },
    { _id: \"Bar\", flag1: true, flag2: false, flag3: true }
]

Answer 1

The aggregate call can be passed a callback function, which is called after the aggregation has completed.

function getValuesAndMessges( params, callback ) {

  db.collection.aggregate([
    { "$project": {
       "_id": 1,
       "flag1": { "$first": "$flag1" },
       "flag2": { "$first": "$flag2" },
       "flag3": { "$first": "$flag3" },
    }}
  ], function( err, results ) {
    if ( !err ) {
      result.forEach( result => {
        // process items in results here, setting a value
        // using the actual logic for writing message ...
        if( flag1 )
          result.message = "broken";
        else
          result.messsge = 'OK';
      });
    }
    callback(err, results);
  });
}

this way each of your aggregated items (based on your conditions / parameters) will have a message property (or whatever properties you choose to write) set and you can use them in your calling function.