How to retrieve the value of a specific field using mongo cxx driver

Question

Say, I inserted the following document using the mongo command line or shell:

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

Then this entry is logged into the database with a system created ID.

If I want to achieve the following command line which only returns the value of a specific filed (_id in this case), using the cxx driver how should I do it?

Here is the command line:

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

I tried the following C++ code

 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;
 }

It simply give me nothing.

If I set

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

Then it returns the entire JSON message for me.

Please let me know if you have any better ideas.

You are including the projection part in the query, which won't work. Provide a ```mongocxx::options::find``` object as the second argument to ```mongocxx::collection::find``` which has been configured by calling the ```mongocxx::options::find::projection``` method. — acm, Aug 23 '16 at 12:53
Thanks for your reply, can you please show me some example or links? — Joe, Aug 23 '16 at 13:07

score 10 · Accepted Answer · answered Aug 23 '16 at 13:40

Here's an example:

#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;
  }
}

Thank you very much for your answer sir. I see that you are a software engineer from mongo db, is it possible that you can suggest mongo putting more examples to the cxx driver tutorial/documentation? — Joe, Aug 23 '16 at 20:38
I found the "finalize" somewhat buggy. If I put finalize at the end, it returns everything. However, if I remove the finalize, it returns only "_id", which is the expected behavior. You might also want to use document.view() in opts.projection(). Anyway thanks a lot for your help. — Joe, Aug 24 '16 at 03:07
Which "finalize" did you have problem with? I tested the code I posted and it worked as expected. (Admittedly, I tested it against the master branch.) — xdg, Aug 26 '16 at 13:09
`bsoncxx::builder::stream::finalize`... well it doesn't matter, probably I used some wrong namespace or it's due to my env, or I did something wrong, etc. I don't wanna spend time debug this as long as my code is working without `finalize`. Thanks for your help :) — Joe, Aug 26 '16 at 20:05

How to retrieve the value of a specific field using mongo cxx driver

1 Answers1