Overview
Within a collection, different documents might contain different values for a single field. For example, one document in the restaurant collection has a borough value of "Manhattan", and another has a borough value of "Queens". With the Java Reactive Streams driver, you can retrieve all the distinct values that a field contains across multiple documents in a collection.
Sample Data
The examples in this guide use the sample_restaurants.restaurants collection from the Atlas sample datasets. To learn how to create a free MongoDB Atlas cluster and load the sample datasets, see Get Started.
Important
Project Reactor Library
This guide uses the Project Reactor library to consume Publisher instances returned by the Java Reactive Streams driver methods. To learn more about the Project Reactor library and how to use it, see Getting Started in the Reactor documentation. To learn more about how we use Project Reactor library methods in this guide, see the Write Data to MongoDB 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:
DistinctPublisher<String> distinctPublisher = collection .distinct("borough", String.class); Flux.from(distinctPublisher) .doOnNext(System.out::println) .blockLast();
Bronx Brooklyn Manhattan Missing Queens Staten Island
The results show every distinct value that appears in the borough field across all documents in the collection. 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 Specify a Query.
The following example retrieves the distinct values of the borough field for all documents that have a cuisine field value of "Italian":
Bson filter = Filters.eq("cuisine", "Italian"); DistinctPublisher<String> distinctPublisher = collection .distinct("borough", String.class) .filter(filter); Flux.from(distinctPublisher) .doOnNext(System.out::println) .blockLast();
Bronx Brooklyn Manhattan Queens Staten Island
Modify Distinct Behavior
The distinct() method can be modified by chaining methods to the distinct() method call. If you don't specify any options, the driver does not customize the operation.
The following table describes some methods you can use to customize the distinct() operation:
Method | Description |
|---|---|
| Sets the number of documents to return per batch. By default, the driver sets this value to
A |
| Specifies the kind of language collation to use when sorting results. For more information, see Collation in the MongoDB Server manual. |
| Specifies a comment to attach to the operation. |
| Sets the query filter to apply to the query. |
For a complete list of methods you can use to modify the distinct() method, see the DistinctPublisher API documentation.
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 uses the comment option to add a comment to the operation.
Bson filter = Filters.and( Filters.eq("borough", "Bronx"), Filters.eq("cuisine", "Pizza") ); DistinctPublisher<String> distinctPublisher = collection .distinct("name", String.class) .filter(filter) .comment("Bronx pizza restaurants"); Flux.from(distinctPublisher) .doOnNext(System.out::println) .blockLast();
$1.25 Pizza 18 East Gunhill Pizza 2 Bros Aenos Pizza Alitalia Pizza Restaurant ...
Additional Information
To learn more about the distinct command, see the Distinct 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: