docs: def merge_join_docs(self, keys):
          """
              Merges the final list of docs
              :param left_collection_list: 
              :type  left_collection_list: MongoCollection
  
              :return join: dict
          """
  
          join = defaultdict(list)
  
          for key in keys:
              join[key] = self.generate_join_docs_list(
                  self.collections_data['left'].get(key, []), self.collections_data['right'].get(key, []))
          return join