code: def _serialize_to_many(self, key, vals, rlink):
          
  
          rel = {
              key: {
                  'data': [],
                  'links': {
                      'related': rlink + '/' + key
                  }
              }
          }
  
          try:
              for val in vals:
                  rel[key]['data'].append({
                      'id': val['rid'],
                      'type': val['rtype'],
                  })
          except TypeError:
              del rel[key]['data']
  
          return rel
docs: Make a to_many JSON API compliant
  
          :spec:
              jsonapi.org/format/#document-resource-object-relationships
          :param key:
              the string name of the relationship field
          :param vals:
              array of dict's containing `rid` & `rtype` keys for the
              to_many, empty array if no values, & None if the to_manys
              values are unknown
          :return:
              dict as documented in the spec link