code: def dump_graph(self):
          
          # we have to hold the lock for the entire dump, if other threads modify
          # self.relations or any cache entry's referenced_by during iteration
          # it's a runtime error!
          with self.lock:
              return {
                  dot_separated(k): v.dump_graph_entry()
                  for k, v in self.relations.items()
              }
docs: Dump a key-only representation of the schema to a dictionary. Every
          known relation is a key with a value of a list of keys it is referenced
          by.