如何使用蒙戈CXX司机

Question

说检索特定字段的值，我插使用蒙戈命令行或shell下列文件：

db.Users.insert( 
    { 
     "user info":{ 
      "user name" : "Joe", 
      "password" : "!@#%$%" , 
      "Facebook" : "aaa", 
      "Google" : "joe z" 
     } 
    } 
) 

那么这个输入登录到与创建的系统数据库ID。

如果我想实现下面的命令行，它只返回特定字段的值（在这种情况下是_id），使用cxx驱动程序我应该怎么做？

这是命令行：

db.Users.find({"user info.user name": "Joe"}, {"_id":1}) 

我尝试下面的C++代码

bsoncxx::builder::stream::document document{} ; 
document<<"user info.user name"<<"Joe"<<"_id"<<1; 
auto cursor = myCollection.find(document.view()); 

for (auto && doc : cursor) { 
    std::cout << bsoncxx::to_json(doc) << std::endl; 
} 

它只是给我什么。

如果我设置

document<<"user info.user name"<<"Joe" 

然后返回整个JSON消息给我。

请让我知道你是否有更好的点子。

Answer 1

下面是一个例子：

#include <iostream> 

#include <bsoncxx/builder/stream/document.hpp> 
#include <bsoncxx/json.hpp> 

#include <mongocxx/client.hpp> 
#include <mongocxx/options/find.hpp> 
#include <mongocxx/instance.hpp> 
#include <mongocxx/uri.hpp> 

using bsoncxx::builder::stream::document; 
using bsoncxx::builder::stream::open_document; 
using bsoncxx::builder::stream::close_document; 
using bsoncxx::builder::stream::finalize; 

int main(int, char **) { 
    mongocxx::instance inst{}; 
    mongocxx::client conn{mongocxx::uri{}}; 

    auto coll = conn["test"]["foo"]; 
    coll.drop(); 

    // Insert a test document 
    auto joe = document{} << "user info" << open_document << "user name" 
         << "Joe" << close_document << finalize;  
    auto result = coll.insert_one(joe.view()); 
    std::cout << "Inserted " << result->inserted_id().get_oid().value.to_string() 
      << std::endl; 

    // Create the query filter 
    auto filter = document{} << "user info.user name" 
          << "Joe" << finalize; 

    // Create the find options with the projection 
    mongocxx::options::find opts{}; 
    opts.projection(document{} << "_id" << 1 << finalize); 


    // Execute find with options 
    auto cursor = coll.find(filter.view(), opts); 
    for (auto &&doc : cursor) { 
    std::cout << bsoncxx::to_json(doc) << std::endl; 
    } 
}