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Read Data

Specify Fields To Return

On this page

  • Overview
  • Sample Data
  • Projection Types
  • Specify Fields to Include
  • Exclude the _id Field
  • Specify Fields to Exclude
  • Additional Information
  • API Documentation

Overview

In this guide, you can learn how to use the C++ driver to specify which fields to return from a read operation by using a projection. A projection is a document that specifies which fields MongoDB returns from a query.

Sample Data

The examples in this guide use the restaurants collection in the sample_restaurants database from the Atlas sample datasets. To access this collection from your C++ application, instantiate a mongocxx::client that connects to an Atlas cluster and assign the following values to your db and collection variables:

auto db = client["sample_restaurants"];
auto collection = db["restaurants"];

To learn how to create a free MongoDB Atlas cluster and load the sample datasets, see the Get Started with Atlas guide.

Projection Types

You can use a projection to specify which fields to include in a return document, or to specify which fields to exclude. You cannot combine inclusion and exclusion statements in a single projection, unless you are excluding the _id field.

Specify Fields to Include

To specify the fields to include in the result, create an instance of the mongocxx::options::find class and set its projection field. To set this field, use the following syntax:

<options instance>.projection(make_document(kvp("<field name>", 1)));

The following example sets the projection field of a mongocxx::options::find object to return only the name, cuisine, and borough fields of matching documents. It then calls the find() method to find all restaurants in which the name field value is "Emerald Pub", passing the mongocxx::options::find object as a parameter to find():

mongocxx::options::find opts{};
opts.projection(make_document(kvp("name", 1), kvp("cuisine", 1), kvp("borough", 1)));
auto cursor = collection.find(make_document(kvp("name", "Emerald Pub")), opts);
for(auto&& doc : cursor) {
    std::cout << bsoncxx::to_json(doc) << std::endl;
}
{ "_id" : { "$oid" : "..." }, "borough" : "Manhattan", "cuisine" : "American", "name" : "Emerald Pub" }
{ "_id" : { "$oid" : "..." }, "borough" : "Queens", "cuisine" : "American", "name" : "Emerald Pub" }

When you use a projection to specify fields to include in the return document, the _id field is also included by default. All other fields are implicitly excluded. To remove the _id field from the return document, you must explicitly exclude it.

Exclude the _id Field

When specifying fields to include, you can also exclude the _id field from the returned document.

The following example performs the same query as the preceding example but excludes the _id field from the projection:

mongocxx::options::find opts{};
opts.projection(make_document(kvp("_id", 0), kvp("name", 1), kvp("cuisine", 1), kvp("borough", 1)));
auto cursor = collection.find(make_document(kvp("name", "Emerald Pub")), opts);
for(auto&& doc : cursor) {
    std::cout << bsoncxx::to_json(doc) << std::endl;
}
{ "borough" : "Manhattan", "cuisine" : "American", "name" : "Emerald Pub" }
{ "borough" : "Queens", "cuisine" : "American", "name" : "Emerald Pub" }

Specify Fields to Exclude

To specify the fields to exclude in the result, create an instance of the mongocxx::options::find class and set its projection field. To set this field, use the following syntax:

<options instance>.projection(make_document(kvp("<field name>", 0)));

The following example sets the projection field of a mongocxx::options::find object to exclude the grades and address fields of matching documents. It then calls the find() method to find all restaurants in which the name field value is "Emerald Pub", passing the mongocxx::options::find object as a parameter to find():

mongocxx::options::find opts{};
opts.projection(make_document(kvp("grades", 0), kvp("address", 0)));
auto cursor = collection.find(make_document(kvp("name", "Emerald Pub")), opts);
for(auto&& doc : cursor) {
    std::cout << bsoncxx::to_json(doc) << std::endl;
}
{ "_id" : { "$oid" : "..." }, "borough" : "Manhattan", "cuisine" : "American", "name" : "Emerald Pub", "restaurant_id" : "40367329" }
{ "_id" : { "$oid" : "..." }, "borough" : "Queens", "cuisine" : "American", "name" : "Emerald Pub", "restaurant_id" : "40668598" }

When you use a projection to specify which fields to exclude, any unspecified fields are implicitly included in the return document.

Additional Information

To learn more about projections, see the Project Fields guide in the MongoDB Server manual.

API Documentation

To learn more about any of the methods or types discussed in this guide, see the following API documentation:

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