Docs Menu

Docs HomeDevelop ApplicationsMongoDB DriversKotlin Coroutine

Retrieve Distinct Values of a Field

You can retrieve a list of distinct values for a field across a collection by calling the distinct() method on a MongoCollection object. Pass the document field name as the first parameter and the class you want to cast the results to as the type parameter.

The following snippets demonstrate the distinct() method using the movies collection in the sample_mflix sample database. Documents are modeled with the following Kotlin data class:

data class Movie(
val type: String,
val languages: List<String>,
val countries: List<String>,
val awards: Awards){
data class Awards(val wins: Int)
}

The following method call returns each distinct value of the countries field in the movies collection:

collection.distinct<String>(Movie::countries.name)

You can specify a field on the document or one within an embedded document using dot notation. The following method call returns each distinct value of the wins field in the awards embedded document:

collection.distinct<Int>("${Movie::awards.name}.${Movie.Awards::wins.name}")

You can also limit the set of documents from which your MongoDB instance retrieves distinct values with a query filter as a second parameter, as follows:

collection.distinct<String>(Movie::type.name, Filters.eq(Movie::languages.name, "French"))

The distinct() method returns an object that implements the DistinctFlow class, which contains methods to access, organize, and traverse the results. DistinctFlow delegates to the Flow interface from the Kotlin Coroutines library, allowing access to methods such as first() and firstOrNull().

For more information, see our guide on Accessing Data From a Flow.

The following example retrieves a list of distinct values for the year document field from the movies collection. It uses a query filter to match movies that include "Carl Franklin" as one of the values in the directors array.

When you run the example, you should see output that reports each distinct year for all the movies that Carl Franklin was included as a director.

Note

This example connects to an instance of MongoDB using a connection URI. To learn more about connecting to your MongoDB instance, see the connection guide.

import com.mongodb.MongoException
import com.mongodb.client.model.Filters
import com.mongodb.kotlin.client.coroutine.MongoClient
import kotlinx.coroutines.runBlocking
data class Movie(val year: Int, val directors: List<String>)
fun main() = runBlocking {
// Replace the uri string with your MongoDB deployment's connection string
val uri = "<connection string uri>"
val mongoClient = MongoClient.create(uri)
val database = mongoClient.getDatabase("sample_mflix")
val collection = database.getCollection<Movie>("movies")
try {
val resultsFlow = collection.distinct<Int>(
Movie::year.name, Filters.eq(Movie::directors.name, "Carl Franklin")
)
resultsFlow.collect { println(it) }
} catch (e: MongoException) {
System.err.println("An error occurred: $e")
}
mongoClient.close()
}

For additional information on the classes and methods mentioned on this page, see the following resources:

  • distinct() API Documentation

  • distinctFlow API Documentation

  • Dot Notation Server Manual Entry

←  Count DocumentsRun a Command →