Retrieve Distinct Values

Overview

In this guide, you can learn how to retrieve distinct values for a specified field across a single collection.

Sample Data

The example in this guide uses the following Course struct as a model for documents in the courses collection:

type Course struct {
	Title      string
	Department string
	Enrollment int32
}

To run the example, load the sample data into the db.courses collection with the following snippet:

coll := client.Database("db").Collection("courses")
docs := []any{
	Course{Title: "World Fiction", Department: "English", Enrollment: 35},
	Course{Title: "Abstract Algebra", Department: "Mathematics", Enrollment: 60},
	Course{Title: "Modern Poetry", Department: "English", Enrollment: 12},
	Course{Title: "Plate Tectonics", Department: "Geology", Enrollment: 30},
}
result, err := coll.InsertMany(context.TODO(), docs)

Tip

Nonexistent Databases and Collections

If the necessary database and collection don't exist when you perform a write operation, the server implicitly creates them.

Each document contains a description of a university course that includes the course title, department, and enrollment. These items correspond to the title, department, and enrollment fields in each document.

Distinct

To retrieve distinct values for a specified field across a single collection, pass the following parameters to the Distinct() method:

The field name for which you want to retrieve the distinct values
A non-nil query filter specifying which documents to match

Tip

If you specify an empty query filter, the Distinct() method searches for distinct values across all documents in a collection.

Modify Behavior

You can modify the behavior of the Distinct() method by passing in a DistinctOptions. If you don't specify a DistinctOptions, the driver uses the default values for each option.

The DistinctOptions type allows you to configure options with the following methods:

Method	Description
`SetCollation()`	The type of language collation to use when sorting results. Default: `nil`
`SetComment()`	Sets a comment to attach to the distinct operation. Default: `nil`

Example

The following example matches documents with an enrollment field value less than 50 and prints the distinct values of the department field by using the Distinct() method:

filter := bson.D{{"enrollment", bson.D{{"$lt", 50}}}}
var arr []string
err = coll.Distinct(context.TODO(), "department", filter).Decode(&arr)
if err != nil {
	panic(err)
}
fmt.Printf("%s\n", arr)

Retrieve Distinct Values Example: Full File

Note

Example Setup

This example connects to an instance of MongoDB by using a connection URI. To learn more about connecting to your MongoDB instance, see Create a MongoClient. This example also uses the restaurants collection in the sample_restaurants database included in the Atlas sample datasets. You can load them into your database on a free cluster on MongoDB Atlas by following the steps in Get Started with Atlas.

This example performs the following actions on the restaurant collection:

Matches documents in which the value of the cuisine field is "Tapas"
Returns distinct values of the borough field from the matched documents

// Retrieves distinct values of a field by using the Go driver
package main
import (
	"context"
	"fmt"
	"log"
	"os"
	"github.com/joho/godotenv"
	"go.mongodb.org/mongo-driver/v2/bson"
	"go.mongodb.org/mongo-driver/v2/mongo"
	"go.mongodb.org/mongo-driver/v2/mongo/options"
)
type Restaurant struct {
	ID           bson.ObjectID `bson:"_id"`
	Name         string
	RestaurantId string `bson:"restaurant_id"`
	Cuisine      string
	Address      interface{}
	Borough      string
	Grades       interface{}
}
func main() {
	if err := godotenv.Load(); err != nil {
		log.Println("No .env file found")
	}
	var uri string
	if uri = os.Getenv("MONGODB_URI"); uri == "" {
		log.Fatal("You must set your 'MONGODB_URI' environment variable. See\n\t https://www.mongodb.com/ja-jp/docs/drivers/go/current/usage-examples/#environment-variable")
	}
	client, err := mongo.Connect(options.Client().ApplyURI(uri))
	if err != nil {
		panic(err)
	}
	defer func() {
		if err = client.Disconnect(context.TODO()); err != nil {
			panic(err)
		}
	}()
	// Filters the collection for documents where the value of cuisine is "Tapas"
	coll := client.Database("sample_restaurants").Collection("restaurants")
	filter := bson.D{{"cuisine", "Tapas"}}
	// Retrieves the distinct values of the "borough" field in documents
	// that match the filter
	var arr []string
	err = coll.Distinct(context.TODO(), "borough", filter).Decode(&arr)
	if err != nil {
		panic(err)
	}
	// Prints the distinct "borough" values
	for _, result := range arr {
		fmt.Println(result)
	}
	// When you run this file, it should print:
	// Brooklyn
	// Manhattan
	// Queens
}

Additional Information

To learn about constructing a query filter, see Specify a Query.

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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