How to extract data from c++ driver cursor

Working with Data Drivers
2

Hi @arif_saeed,

The cursor iterates though document::view, so you can retrieve a document::element from the view using the operator[]() member of view. For example:

auto cursor = collection.find({});
for (auto&& doc : cursor) {
    bsoncxx::document::element curvename = doc["curveName"];
    std::cout << "curve name: " << curvename.get_utf8().value << std::endl;
}

Please note that by default MongoDB queries return all fields in matching documents. To limit the amount of data returned to the application you can specify projection to restrict fields to return. See Project Fields to Return from Query for more information. For example in C++ to project just curveName field:

mongocxx::options::find opts;
opts.projection(make_document(kvp("curveName", 1)));
auto cursor = collection.find({}, opts);

Looking at the example line that you would like to do, I assumed that you’re trying to retrieve curve names from the collection and store into an array. If so, you can try using aggregation pipeline. For example, to retrieve all curveName field values from the collection into an array with no duplicate:

#include <bsoncxx/builder/basic/kvp.hpp>
#include <bsoncxx/builder/basic/document.hpp>

using bsoncxx::builder::basic::make_document;
using bsoncxx::builder::basic::kvp;

mongocxx::pipeline p{};
p.group(make_document(
    kvp("_id", NULL),
    kvp("curveNames", make_document(kvp("$addToSet", "$curveName")))
));
auto cursor = collection.aggregate(p, mongocxx::options::aggregate{});

See also $push operator, which adds array field without removing duplicates.

Regards,
Wan.

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