如何为客户端更改mongocxx文件？

Question

auto cur = collection.find(make_document(kvp("userId", uid)), findOptions);
        auto collects = bsoncxx::builder::basic::array{};

        for (auto &&doc : cur) {
            doc["time"] = "some new value";  // this line can't execute
            collects.append(doc);
        }
response_success(req, collects);

我想更改doc time字段值，以便将收集返回给客户端。但我发现没有接口可以从mongocxx调用。

谁能告诉我该怎么办？谢谢！