Hello, Thanks for responding. ^^

i didn't want to make any changes to the id field.

i have a user struct like this.

type User struct {
	ID         primitive.ObjectID `bson:"_id,omitempty" json:"id"`
	FirstName  string             `bson:"firstname" json:"firstname"`
	LastName   string             `bson:"lastname" json:"lastname"`
	Email      string             `bson:"email" index:"email" json:"email"`
 }

i wanted the Email field to be unique, just like the ID. so when a new user tries to create an account with the same email, it'll return an error.

it isn't a problem anymore because i figured out a way... i created a custom struct tag that validates whether a user with the same email exist in the database.