Additional "id" field generated when posting into MongoDB

I'm learning Go and Gin framework.
I built a simple microservice connected to a MongoDB collection, everything works but when I add a document using a POST it adds the "id" field instead of generating the key "_id" field, is there a way to avoid this?

This is my func:

func (r *Rest) CreatePost(c *gin.Context) {
var postCollection = database.GetCollection(r.db, "GoDB")
ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
post := new(model.Post)
defer cancel()

if err := c.ShouldBindJSON(&post); err != nil {
	c.JSON(http.StatusBadRequest, gin.H{"message": err})
	log.Fatal(err)
	return
}

// validation
if err := post.Validate(); err == nil {
	c.JSON(http.StatusOK, gin.H{"input": "valid"})
} else {
	c.JSON(http.StatusBadRequest, gin.H{"input validation": err.Error()})
	return
}

postPayload := model.Post{
	ID:      primitive.NewObjectID(),
	Title:   post.Title,
	Article: post.Article,
}

result, err := postCollection.InsertOne(ctx, postPayload)

if err != nil {
	c.JSON(http.StatusInternalServerError, gin.H{"message": err})
	return
}

c.JSON(http.StatusOK, gin.H{"message": "Posted succesfully", "Data": 
map[string]interface{}{"data": result}})
}

This is my model:

type Post struct {
ID      primitive.ObjectID
Title   string `validate:"required,gte=2,lte=20"`
Article string `validate:"required,gte=4,lte=40"`
}

By default, the key for ID is id. You should use the bson tag to generate the key _id instead.

type Post struct {
	ID      primitive.ObjectID `bson:"_id"`
	Title   string             `validate:"required,gte=2,lte=20"`
	Article string             `validate:"required,gte=4,lte=40"`
}

Here is the doc:

> When marshalling a struct, each field will be lowercased to generate the key for the corresponding BSON element. For example, a struct field named "Foo" will generate key "foo". This can be overridden via a struct tag (e.g. bson:"fooField" to generate key "fooField" instead).

When the document does not contain an element named _id, the driver will add one automatically (see the source code):

// ensureID inserts the given ObjectID as an element named "_id" at the
// beginning of the given BSON document if there is not an "_id" already. If
// there is already an element named "_id", the document is not modified. It
// returns the resulting document and the decoded Go value of the "_id" element.
func ensureID(
	doc bsoncore.Document,
	oid primitive.ObjectID,
	bsonOpts *options.BSONOptions,
	reg *bsoncodec.Registry,
) (bsoncore.Document, interface{}, error) {

Here is a demo:

package main

import (
	"context"

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

type Post struct {
	ID      primitive.ObjectID `bson:"_id"`
	Title   string             `validate:"required,gte=2,lte=20"`
	Article string             `validate:"required,gte=4,lte=40"`
}

func main() {
	client, err := mongo.Connect(context.Background(), options.Client().ApplyURI("mongodb://localhost"))
	if err != nil {
		panic(err)
	}

	postCollection := client.Database("demo").Collection("posts")
	post := Post{
		ID:      primitive.NewObjectID(),
		Title:   "test title",
		Article: "test content",
	}
	if err != nil {
		panic(err)
	}

	if _, err = postCollection.InsertOne(context.Background(), post); err != nil {
		panic(err)
	}
}

And the document created in the database:

demo> db.posts.find()
[
  {
    _id: ObjectId("64a53bcbb7be31ae42e6c00c"),
    title: 'test title',
    article: 'test content'
  }
]