code: function save(data) {
              this.clientData = _.cloneDeep(data) || {};
              this.serverData = _.cloneDeep(this.attributes);
  
              return ParentModel.prototype.save.apply(this, arguments);
          }
docs: We have to remember current server data and client data.
  The `sync` method has no access to it.
  `updated_at` is already set to "Date.now" when the overridden `sync.update` is called.
  See https://github.com/tgriesser/bookshelf/blob/79c526870e618748caf94e7476a0bc796ee090a6/src/model.js#L955