code: def get_one(self, key):
          
          query = build_db_query(self.primary_key, key)
          collection = self.ds.connection(self.collection_name)
  
          document = collection.find_one(query)
          if document is None:
              raise LookupError('{0} with key {1} was not found'.format(self.model_klass.__name__, query))
          return self.model_klass.from_json(document)
docs: method finds single record base on the given primary key and returns it to the caller