How can I fill and fill in mongoose

I am having trouble filling in "Main", collecting, grouping works very well, but I really don't know how to fill or even .find after aggregating. I suppose I am doing model casting here or so:

Main.aggregate([
      {$match : query},
      {
        $group:{
          _id: queryGroupBy,
          activated: {$sum: '$activated'},
          componentTitle: {$first:'$componentTitle'},
          titlePrefix: {$first:'$titlePrefix'},
          operator_name: {$first:'$operator_name'}
        }
      },
      {
        $project:{
          _id: '$_id',
          summation: '$activated',
          componentTitle: '$componentTitle',
          titlePrefix: '$titlePrefix',
          operator_name: '$operator_name'   
        }
      }],
      function(err,results) {
        if (err) throw err;
        result = results.map(function(doc) { 
          doc._id = doc._id,
          doc.activated = doc.activated,
          doc.componentTitle = doc.componentTitle,
          doc.titlePrefix = doc.titlePrefix,
          doc.operator_name = doc.operator_name,
          doc.fssStatusFDD = "",
          doc.dateUpdated = "",
          delete doc._id;
          delete doc.summation;

            var _main = new Main();
            _main = doc;
            console.log('test3');
            return _main
            });
        Main.populate( results, { "path": "operator_name" }, function(err,results) {
            if (err) throw err;
            console.log( JSON.stringify( results, undefined, 4 ) );
        });

        console.log('good');
        return res.send(results);
    });

      

        
        
        
      

    

Any suggestion is greatly appreciated and appreciated.