Wie erstelle ich einen eindeutigen Paarindex für Mongodb?

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
}

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

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

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

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

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)
}