如何檢查是否GQL對象爲空

Question

我想檢查是否有GQL對象是否爲空（在使用Python谷歌應用程序引擎），我應該怎麼辦呢？ 我試過，但沒有奏效：

comments = db.GqlQuery("SELECT * " 
           "FROM Comment " 
           "where ANCESTOR IS :1 " 
           "order by date DESC", 
           movie_data) 
     if comments: 
      ratingsum = 0 
      i=0 
      for comment in comments: 
       ratingsum=ratingsum + comment.rating 
       i+=1 
      average = ratingsum/i 
     else: 
      average = "no ratings" 

Answer 1

的GQL查詢爲您提供了一個查詢對象，而不是結果（實體）。要獲得結果，您必須獲取結果：使用fetch，count，get或遍歷結果集。

Answer 2

According to @NickJohnson，呼籲count將需要執行相同的查詢兩次。因此，爲了避免這種情況，因爲你將不得不通過評論進行迭代，你還不如「檢測」當評論是空的，通過觀察副作用 - 這i將是零，如果comments爲空：

comments = db.GqlQuery("SELECT * " 
           "FROM Comment " 
           "where ANCESTOR IS :1 " 
           "order by date DESC", 
           movie_data) 
ratingsum = 0 
i = 0 
for comment in comments: 
    ratingsum += comment.rating 
    i += 1 
if i:  
    average = ratingsum/i 
else:  
    average = "no ratings"