code: def group_by_source(self):
          
          from collections import defaultdict
          docs = defaultdict(list)
  
          for k, v in self.items():
              if 'source' in v:
                  docs[v.source].append(dict(v.items()))
  
          return docs
docs: Return a dict of all of the docs, with the source associated
          with the doc as a key