{
  $group: {
     averageDividend: {
        $sum: {
          $multiply: ['$score', '$field.weight'],
        }
    },
    averageDivisor: {
       $sum: '$field.weight'
    },
  }
}

import { sum, multiply } from '@entria/mongo-aggregate-lodash';

{
  $group: {
     averageDividend: sum(multiply('$score', '$field.weight')),
      averageDivisor: sum('$field.weight'),
  }
}