void Insert(std::unique_ptr<T> object, int node_id) {
    objects_by_id_[node_id] = object.get();
    Insert(std::move(object));
  }

  // Find an object by the object graph node id.
  // Returns nullptr if there is no such object.
  T* Find(int node_id) {
    auto it = objects_by_id_.find(node_id);
    return it == objects_by_id_.end() ? nullptr : it->second;
  }

 private: