how to register the id of a different collection

I am doing a practice of an application for the sale and creation of video games, the users of the application will have multiple roles which are assigned only by the administrator, for that I have two collections, one where all the roles will be stored and another for the users , but I am encountering problem and it is how to get the id of the roles, the only way I can think of is to have a portion of string in my user model that saves the id by searching by means the role name, but not in this way is correct because it does not save the ID of the object.<br/>
This my model role and user:

type Role struct {
ID        primitive.ObjectID `bson:"_id,omitempty" json:"id"`
Name      string             `bson:"name,omitempty"`
State     bool               `bson:"state,omitempty"`
CreatedAt time.Time          `bson:"created_at,omitempty" json:"created_at"`
UpdatedAt time.Time          `bson:"updated_at,omitempty" json:"updated_at"`
}
type User struct {
ID        primitive.ObjectID `bson:"_id,omitempty" json:"id"`
FirstName string             `bson:"first_name,omitempty" json:"first_name"`
LastName  string             `bson:"last_name,omitempty" json:"last_name"`
Email     string             `bson:"email,omitempty" json:"email"`
Password  string             `bson:"Password,omitempty" json:"Password"`
Avatar    string             `bson:"avatar,omitempty" json:"avatar"`
Role      []string           `bson:"role,omitempty" json:"role"`
CreatedAt time.Time          `bson:"created_at,omitempty" json:"created_at"`
UpdatedAt time.Time          `bson:"updated_at,omitempty" json:"updated_at"`
}

This my create user:

func CreateUser(w http.ResponseWriter, r *http.Request) {
w.Header().Set("Content-Type", "application/json")
ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
defer cancel()
collection := storage.Connection().Database("testing")
collectionRole := collection.Collection("roles")
collectionUser := collection.Collection("users")
u := &model.User{}
if err := json.NewDecoder(r.Body).Decode(u); err != nil {
http.Error(w, "format incorrect", http.StatusBadRequest)
return
}
var rl []model.Role
var roles []string
cursor, err := collectionRole.Find(ctx, bson.D{{"name", bson.D{{"$in", u.Role}}}})
if err != nil {
http.Error(w, "collection not found ", 404)
return
}
if err := cursor.All(ctx, &rl); err != nil {
log.Fatal(err.Error())
return
}
for _, rol := range rl {
roles = append(roles, rol.ID.Hex())
}
result, err := collectionUser.InsertOne(ctx, bson.D{
{"first_name", u.FirstName},
{"role", roles},
})
if err != nil {
log.Fatal(err.Error())
return
}
json.NewEncoder(w).Encode(result)
}

> a portion of string in my user model that saves the id by searching by means the role name, but not in this way is correct because it does not save the ID of the object.

Try storing the ObjectID instead of the role string name. As this would allow you to alter the role's name without having to update all of the users collection.

You could either store the primitive.ObjectID as is, or store the hex string using ObjectID.Hex() and ObjectIDFromHex() methods.