Get populated data from subdocument excluding specific fields

gistfile1.js

var populate_data_from = 'trainers tags',
    video_data = {
        path: 'videos',
        model: 'Video',
        select: 'title description legacy_video videofiles.sources.10'
    },
    excludes = '-royalties';
    
/**
 * @return {object} A single collection with videos, tags and trainers
 * populated based on a partnumber
 */
VideoCollection.statics.FindByPartNumber = function(partnumber, callback) {
    this.findOne({'partnumber': partnumber}).select(excludes).populate(populate_data_from).populate(video_data).exec(callback);
};