指定要返回的字段

样本数据

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"];

要学习如何创建免费的MongoDB Atlas 集群并加载示例数据集，请参阅MongoDB 入门指南。

指定要包含的字段

要指定结果中包含的字段，请创建mongocxx::options::find类的实例并设立其projection字段。 要设立此字段，请使用以下语法：

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

以下示例将mongocxx::options::find对象的projection字段设置为仅返回匹配文档的name 、 cuisine和borough字段。 然后，它调用find()方法查找name字段值为"Emerald Pub"的所有餐厅，并将mongocxx::options::find对象作为参数传递给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.

排除 _id 字段

在指定要包含的字段时，您还可以从返回的文档中排除_id字段。

以下示例执行与上一示例相同的查询，但从投影中排除_id字段：

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" }

指定要排除的字段

要指定要在结果中排除的字段，请创建mongocxx::options::find类的实例并设立其projection字段。 要设立此字段，请使用以下语法：

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

以下示例将mongocxx::options::find对象的projection字段设置为排除匹配文档的grades和address字段。 然后，它调用find()方法查找name字段值为"Emerald Pub"的所有餐厅，并将mongocxx::options::find对象作为参数传递给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" }

当您使用投影指定要排除的字段时，任何未指定的字段都将隐式包含在返回文档中。

API 文档

要进一步了解本指南所讨论的任何方法或类型，请参阅以下 API 文档：

• find()

• mongocxx::options::find

• 投影