Mongoose subdocument not created in mongoDB but added to parent array

I have a method to update the commerce information. There is the possibility to add Workdays to the commerce by sending them in the request body. There following code works fine except that the workdays are not created in mongoDB. They are only saved in the Commerce document (as an array od ids) but the Collection called "workday" is not createdin mongoDB. Why it's not created?

            if(req.body.workdays){
                var workdays = req.body.workdays;
                var lunch = req.body.lunch.split("_");

                commerce.workdays=[];
                for(var i =0, size=workdays.length; i<size; i++ ){
                  var item=new Workday();
                  item.dayOfWeek = workdays[i];
                  item.owner=commerce._id;
                  var range = new Range();
                  range.initial = lunch[0];
                  range.end = lunch[1];
                  range.workday = item;
                  item.ranges.push(range);
                  commerce.workdays.push(item);
                }
              } 

              commerce.save(function(err) {
                if(!err) {
                  log.debug('Updated');
                  res.status(200).send(commerce);
                } else {
                  errorHandler.processError(err, res, log);
                }
              });

here are the models:

var CommerceSchema = new Schema({
  // Common fields.
  createdAt  : {type : Date, default : Date.now},
  location: [Number, Number],
  photos: [{type : Schema.Types.ObjectId, ref : 'Photo'}],
  name: { type: String},
  address: { type: String},
  email: { type: String, default: "-"},
  workdays: [{type : Schema.Types.ObjectId, ref : 'Workday'}],
  description: { type: String, default: "-"},
  phone: { type: Number},
  user: {type : String, ref : 'User'},
  type: [{ type: Number, default: 0}]
});

var WorkdaySchema = new Schema({
  dayOfWeek: { type: Number},
  owner: {type : String},
  ranges: [{type : Schema.Types.ObjectId, ref : 'Range'}],
  createdAt  : {type : Date, default : Date.now}
});

var RangeSchema = new Schema({
  initial: { type: Number},
  end: { type: Number},
  workday: {type : String, ref : 'Workday'}
});