How to create an index of pair unique for Mongodb?

I am working with MongoDB, I want to make a pair unique on 2 field.

The below is what I have done so far :

func (repository *TranslationRepository) createIndexes(collection *mongo.Collection) error {
	models := []mongo.IndexModel{
		{
			Keys:    bson.D{{"object_id", 1}, {"object_type", 1}},
			Options: options.Index().SetUnique(true),
		},
		{
			Keys:    bson.D{{"expire_at", 1}},
			Options: options.Index().SetExpireAfterSeconds(0),
		},
	}

	opts := options.CreateIndexes().SetMaxTime(10 * time.Second)
	_, err := collection.Indexes().CreateMany(context.Background(), models, opts)
	return err
}

However when I insert 2 records like this

{
    "object_id"  : "abc",
    "object_type": "SAMPLE" 
}

{
    "object_id"  : "edf",
    "object_type": "SAMPLE" 
}

In the database, there will be only 1 record of

{
    "object_id"  : "edf",
    "object_type": "SAMPLE" 
}

The second already overwrited the first

The below is my sample code of inserting a record

TranslationForm := entity.TranslationForm{
		ObjectID:       "ABC",
		ObjectType:     "SAMPLE",
		SourceLanguage: "en",
		TargetLanguage: "cn",
		Content:        "something",
		ExpireAt:       time.Now(),
	}
res, err := repository.collection.InsertOne(context.TODO(), TranslationForm)

I should have managed your scenario. Let me share a simple program to show off what I achieved.

package main

import (
	"context"
	"fmt"
	"time"

	"go.mongodb.org/mongo-driver/bson"
	"go.mongodb.org/mongo-driver/mongo"
	"go.mongodb.org/mongo-driver/mongo/options"
)

type Object struct {
	ObjectId   string `json:"object_id" bson:"object_id"`
	ObjectType string `json:"object_type" bson:"object_type"`
}

func main() {
	ctx, cancelFunc := context.WithTimeout(context.Background(), time.Second*10)
	defer cancelFunc()

	clientOptions := options.Client().ApplyURI("mongodb://root:root@localhost:27017")
	mongoClient, err := mongo.Connect(ctx, clientOptions)
	if err != nil {
		panic(err)
	}
	defer mongoClient.Disconnect(ctx)

	demoDb := mongoClient.Database("demodb")
	defer demoDb.Drop(ctx)
	myCollection := demoDb.Collection("myCollection")
	defer myCollection.Drop(ctx)

	// create index
	indexModel := mongo.IndexModel{
		Keys: bson.D{
			bson.E{
				Key:   "object_id",
				Value: 1,
			},
			bson.E{
				Key:   "object_type",
				Value: 1,
			},
		},
		Options: options.Index().SetUnique(true),
	}
	idxName, err := myCollection.Indexes().CreateOne(ctx, indexModel)
	if err != nil {
		panic(err)
	}

	fmt.Println("index name:", idxName)

	// delete documents
	defer func() {
		if _, err := myCollection.DeleteMany(ctx, bson.M{}); err != nil {
			panic(err)
		}
	}()

	// insert first doc
	res, err := myCollection.InsertOne(ctx, Object{ObjectId: "abc", ObjectType: "SAMPLE"})
	if err != nil {
		panic(err)
	}
	fmt.Println(res.InsertedID)

	// insert second doc
	// res, err = myCollection.InsertOne(ctx, Object{ObjectId: "abc", ObjectType: "SAMPLE"}) => ERROR
	res, err = myCollection.InsertOne(ctx, Object{ObjectId: "def", ObjectType: "SAMPLE"}) // => OK!
	if err != nil {
		panic(err)
	}
	fmt.Println(res.InsertedID)

	// list all docs
	var objects []Object
	cursor, err := myCollection.Find(ctx, bson.M{})
	if err != nil {
		panic(err)
	}
	if err = cursor.All(ctx, &objects); err != nil {
		panic(err)
	}
	fmt.Println(objects)
}

Now, I'm gonna recap all of the main steps:

1. Definition of the Object struct which is a simplified version of what you need. Please note the bson annotations that are the ones actually used. For the sake of this demo, you can safely omit the json ones.
2. The setup related to the Mongo ecosystem:
   1. Context creation (with a timeout)
   2. Client set up (connection to a local MongoDB instance run via Docker)
   3. Creation of a database called demodb and a collection called myCollection. Plus, I deferred the calls to delete this stuff while exiting the programs (just for clean-up).
3. Creation of a unique compound index on the fields object_id and object_type. Pay attention to the Options field that states the uniqueness of the index with the method SetUnique.
4. Adding documents. Please note that the program won't allow you to insert two documents with the same fields as requested. You can try to comment/uncomment the cases just to have another confirmation.
5. For debug purposes, I finally list the documents in the collection to check whether the second document has been added or not.

I hope that this demo has clarified a few your doubts. Let me know & thanks!