mongo golang driver remove all array items without condition

I need to remove and get all elements of an array in mongodb. I found $pull and $pullAll but they require query condition, how to remove all elements without condition?

The code not work, elements still exist after the $pull:

var UserId = 123
type Event struct {
	UserId uint64 `gorm:"uniqueIndex"`
	Array  [][]byte
}
func main() {
	var DB_NAME = "test"
	var ctx = context.Background()
	client, _ := mongo.Connect(
		ctx, options.Client().ApplyURI("mongodb://localhost"))
	col := client.Database("test").Collection(`Test`)
	{ // pull items
		r := col.FindOneAndUpdate(ctx, bson.M{
			`UserId`: UserId,
		}, bson.M{
			`$pull`: bson.M{
				`Array`: nil,
			},
		}, options.FindOneAndUpdate().SetUpsert(true).SetReturnDocument(options.Before))
		if r.Err() != nil {
			panic(r.Err())
		}
	}
}

Edit:

I found $set': bson.M{"Array": [][]byte{}} does the job, is $pull capable of doing this? Which performance is better?

The $pull operator is a "top level" operator in update statements, so you simply have this the wrong way around:

 r := col.FindOneAndUpdate(ctx, bson.M{bson.M{"$pull": bson.M{"UserId": bson.ObjectIdHex(UserId)}}

The order of update operators is always operator first, action second.

If there is no operator at the "top-level" keys, MongoDB interprets this as just a "plain object" to update and "replace" the matched document. Hence the error about the $ in the key name.