class Bet extends \yii\redis\ActiveRecord
{
    public static function primaryKey()
    {
        return [
                   'game_id', //int
                   'stage_id', //string
                   'user_id' //int
       ];
    }
    // some code
}

        $bet1 = new Bet();
        $bet1->game_id = 1;
        $bet1->user_id = 1;
        $bet1->stage_id = 'flop';
        $bet1->sum = 0;
        $bet1->save();

        $bet2 = Bet::find()->where([
            'game_id' => 1,
            'user_id' => 1,
            'stage_id' => 'flop',
        ])->one();

        $bet2->sum = $bet2->sum + 10;

        $bet2->save(); // not saved, but return true