PriceAlert::find()
    ->joinWith('product')
    ->andWhere(['user_id' => Yii::$app->user->id])
    ->andWhere(['>', 'product.price', 100])
    ->orderBy('complete DESC, updated_at DESC')
    ->all();

或

PriceAlert::find()
    ->joinWith(['product' => function ($q) {
        // add conditions here
    }])
    ->andWhere(['user_id' => Yii::$app->user->id])
    ->orderBy('complete DESC, updated_at DESC')
    ->all();