code: def _add_link(self, referenced_key, dependent_key):
          
          referenced = self.relations.get(referenced_key)
          if referenced is None:
              dbt.exceptions.raise_cache_inconsistent(
                  'in add_link, referenced link key {} not in cache!'
                  .format(referenced_key)
              )
  
          dependent = self.relations.get(dependent_key)
          if dependent is None:
              dbt.exceptions.raise_cache_inconsistent(
                  'in add_link, dependent link key {} not in cache!'
                  .format(dependent_key)
              )
  
          referenced.add_reference(dependent)
docs: Add a link between two relations to the database. Both the old and
          new entries must alraedy exist in the database.
  
          :param _ReferenceKey referenced_key: The key identifying the referenced
              model (the one that if dropped will drop the dependent model).
          :param _ReferenceKey dependent_key: The key identifying the dependent
              model.
          :raises InternalError: If either entry does not exist.