Retrieve Distinct Field Values
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Overview
In this guide, you can learn how to use the Scala 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 Scala 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 Scala application, create a MongoClient that connects to an Atlas cluster
and assign the following values to your database and collection variables:
val database: MongoDatabase = client.getDatabase("sample_restaurants") val collection: MongoCollection[Document] = database.getCollection("restaurants")
To learn how to create a free MongoDB Atlas cluster and load the sample datasets, see the Get Started with Atlas guide.
Retrieve Distinct Values
To retrieve the distinct values for a specified field, call the distinct()
method and pass the name of the field you want to find distinct values for.
Retrieve Values Across a Collection
The following example retrieves the distinct values of the borough field in
the restaurants collection:
collection.distinct("borough") .subscribe((value: String) => println(value), (e: Throwable) => println(s"There was an error: $e"))
Bronx Brooklyn Manhattan Missing Queens Staten Island
The operation returns an instance of the DistinctObservable class, which
you can iterate through to access each distinct borough field value. Although
several documents have the same value in the borough field, each value appears
in the results only once.
Retrieve Values Across Specified Documents
You can provide a query filter to the distinct() method to find distinct
field values within 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":
val filter = equal("cuisine", "Italian") collection.distinct("borough", filter) .subscribe((value: String) => println(value), (e: Throwable) => println(s"There was an error: $e"))
Bronx Brooklyn Manhattan Queens Staten Island
Modify Distinct Behavior
You can modify the behavior of the distinct() method by chaining
methods provided by the DistinctObservable class. The following
table describes some of these methods:
Method | Description |
|---|---|
| Sets the collation to use for the operation. Parameter Type: Collation |
| Sets the maximum amount of time that the operation can run. Parameter Type: Duration |
| Attaches a comment to the operation. Parameter Type: BsonValue or String |
| Retrieves only the first distinct field value. |
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". Then, it chains the comment()
method to distinct() to add a comment to the operation:
val filter = and(equal("borough", "Bronx"), equal("cuisine", "Pizza")) collection.distinct("name", filter) .comment("Bronx Pizza restaurants") .subscribe((value: String) => println(value), (e: Throwable) => println(s"There was an error: $e"))
$1.25 Pizza 18 East Gunhill Pizza 2 Bros Aenos Pizza Alitalia Pizza Restaurant Amici Pizza And Pasta Angie'S Cafe Pizza ...
API Documentation
To learn more about any of the methods or types discussed in this guide, see the following API documentation: