docs: def get_pk_index(self, collection):
          """Return the primary key index for a given collection.
  
          :param collection: the collection for which to return the primary index
  
          :returns: the primary key index of the given collection
  
          """
          cls = self.collections[collection]
  
          if not cls.get_pk_name() in self.indexes[collection]:
              self.create_index(cls.get_pk_name(), collection)
          return self.indexes[collection][cls.get_pk_name()]