code: function toJSON(unfilteredOptions) {
          const options = ghostBookshelf.Model.filterOptions(unfilteredOptions, 'toJSON');
          options.omitPivot = true;
  
          // CASE: get JSON of previous attrs
          if (options.previous) {
              const clonedModel = _.cloneDeep(this);
              clonedModel.attributes = this._previousAttributes;
  
              if (this.relationships) {
                  this.relationships.forEach((relation) => {
                      if (this._previousRelations && this._previousRelations.hasOwnProperty(relation)) {
                          clonedModel.related(relation).models = this._previousRelations[relation].models;
                      }
                  });
              }
  
              return proto.toJSON.call(clonedModel, options);
          }
  
          return proto.toJSON.call(this, options);
      }
docs: `shallow`    - won't return relations
  `omitPivot`  - won't return pivot fields
  
  `toJSON` calls `serialize`.
  
  @param unfilteredOptions
  @returns {*}