code: function findColumnsIndexes(columns, fieldsToShow) {
    if (typeof fieldsToShow === 'string') {
      fieldsToShow = fieldsToShow.split(',');
    }
    var emptyIndexes = [];
    for (var i = 0; i < columns.length; i++) {
      if (fieldsToShow.indexOf(columns[i].path) === -1) {
        emptyIndexes.push(i);
      }
    }
    return emptyIndexes;
  }
docs: function find(model, req) { model.aggregate({userid, last(orderdate), sum(orders)}); //http://stackoverflow.com/questions/25231022/mongoose-how-to-group-by-and-populate //http://stackoverflow.com/questions/31825744/lean-inside-populate-in-mongoose //.populate('userid local.email').exec() }