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Read Data from MongoDB

Retrieve Distinct Field Values

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  • Overview
  • Sample Data
  • distinct() Method
  • Retrieve Distinct Values Across a Collection
  • Retrieve Distinct Values Across Specified Documents
  • Modify Distinct Behavior
  • API Documentation

Overview

In this guide, you can learn how to use the C++ driver to retrieve the distinct values of a specified field across a collection.

Within a collection, different documents might contain different values for a single field. For example, one document in a restaurants collection has a borough value of "Manhattan", and another has a borough value of "Queens". By using the C++ driver, you can retrieve all the unique values that a field contains across multiple documents in a collection.

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.

distinct() Method

To retrieve the distinct values for a specified field, call the distinct() method and pass in the name of the field you want to find distinct values for.

Retrieve Distinct Values Across a Collection

The following example retrieves the distinct values of the borough field in the restaurants collection:

auto cursor = collection.distinct("borough", {});
for(auto&& doc : cursor) {
    std::cout << bsoncxx::to_json(doc) << std::endl;
}
{ "values" : [ "Bronx", "Brooklyn", "Manhattan", "Missing", "Queens", "Staten Island" ],
"ok" : 1.0, "$clusterTime" : { "clusterTime" : { "$timestamp" : { ... } },
"signature" : { "hash" : { ... }, "keyId" : ... } }, "operationTime" : { "$timestamp" : { ... } }

The operation returns a cursor that contains a single document. The document includes a list of distinct borough field values and metadata about the operation. Although several documents have the same value in the borough field, each value appears in the results only once.

Retrieve Distinct Values Across Specified Documents

You can provide a query filter to the distinct() method to find the distinct field values across a subset of documents in a collection. A query filter is an expression that specifies search criteria used to match documents in an operation. For more information about creating a query filter, see the Specify a Query guide.

The following example retrieves the distinct values of the borough field for all documents that have a cuisine field value of "Italian":

auto cursor = collection.distinct("borough", make_document(kvp("cuisine", "Italian")));
for(auto&& doc : cursor) {
    std::cout << bsoncxx::to_json(doc) << std::endl;
}
{ "values" : [ "Bronx", "Brooklyn", "Manhattan", "Queens", "Staten Island" ],
"ok" : 1.0, "$clusterTime" : { "clusterTime" : { "$timestamp" : { ... },
"signature" : { "hash" : { ... }, "keyId" : ... } }, "operationTime" : { "$timestamp" : { ... } }

Modify Distinct Behavior

You can modify the behavior of the distinct() method by passing an instance of the mongocxx::options::distinct class as an argument. The following table describes the fields you can set in a mongocxx::options::distinct instance:

Field
Description
collation
The collation to use for the operation.
Type: bsoncxx::document::view_or_value
max_time
The maximum amount of time in milliseconds that the operation can run.
Type: std::chrono::milliseconds
comment
The comment to attach to the operation.
Type: bsoncxx::types::bson_value::view_or_value
read_preference
The read preference to use for the operation.
Type: mongocxx::read_preference

The following example retrieves the distinct values of the name field for all documents that have a borough field value of "Bronx" and a cuisine field value of "Pizza". It also specifies the comment field of an options instance to add a comment to the operation:

mongocxx::options::distinct opts{};
opts.comment(bsoncxx::types::bson_value::view_or_value{"Bronx pizza restaurants"});
auto cursor = collection.distinct("name",
    make_document(kvp("$and",
                        make_array(make_document(kvp("borough", "Bronx")),
                                    make_document(kvp("cuisine", "Pizza"))))),
    opts
);
for (auto&& doc : cursor) {
    std::cout << bsoncxx::to_json(doc) << std::endl;
}
{ "values" : [ "$1.25 Pizza", "18 East Gunhill Pizza", "2 Bros", "Aenos Pizza", "Alitalia Pizza Restaurant", … ],
  "ok" : 1.0, "$clusterTime" : { "clusterTime" : { … }, "signature" : { … }, "keyId" : … } }, "operationTime" : { … } }

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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Specify Fields To Return