code: function(instance) {
        var loadData = function(data) {
          return _.extend(instance, data);
        };
  
        if (instance.id) {
          // do PUT save
          return api
            .one(config.entity, instance.id)
            .put(instance)
            .then(loadData, console.error);
        } else {
          // no id is set, do a POST
          var endpoint = api;
          if (config.parent) {
            endpoint.one(config.parent.entity, instance[config.parent.key]);
          }
          endpoint.all(config.entity);
  
          return endpoint
            .post(instance)
            .then(loadData, console.error);
        }
      }
docs: Persists entity using rest API
  
  @param {Model} instance
  @return {Promise}