code: def ris(self):
          
          if self.aggregationType != 'Journal':
              raise ValueError('Only Journal articles supported.')
          template = u'''TY  - JOUR
  TI  - {title}
  JO  - {journal}
  VL  - {volume}
  DA  - {date}
  SP  - {pages}
  PY  - {year}
  DO  - {doi}
  UR  - https://doi.org/{doi}
  '''
          ris = template.format(
              title=self.title, journal=self.publicationName,
              volume=self.volume, date=self.coverDate, pages=self.pageRange,
              year=self.coverDate[0:4], doi=self.doi)
          for au in self.authors:
              ris += 'AU  - {}\n'.format(au.indexed_name)
          if self.issueIdentifier is not None:
              ris += 'IS  - {}\n'.format(self.issueIdentifier)
          ris += 'ER  - \n\n'
          return ris
docs: Bibliographic entry in RIS (Research Information System Format)
          format.
  
          Returns
          -------
          ris : str
              The RIS string representing an item.
  
          Raises
          ------
          ValueError : If the item's aggregationType is not Journal.