mongocxx::collection collection = conn["wechat"]["article"];
bsoncxx::builder::stream::document condition, options;
mongocxx::options::find opts;

condition << "subscribeRobot" << open_document << "$exists" << 1 << close_document;
options << "subscribeRobot" << 1;
opts.projection(options.view());
opts.limit(100);

mongocxx::cursor cur = collection.find(condition.view(), opts);
for(auto && doc:cur)