如何检查GQL对象是否为空

Question

我试图检查一个GQL对象是否为空（在使用Python的googleappengine中），我应该怎么做？ 我试过了，但没用：

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

Gql查询提供了一个查询对象，但不提供结果（实体）。要获得结果，必须获取结果：使用fetch、count、get或遍历结果集。在

Answer 2

According to @NickJohnson，调用count将需要执行相同的查询两次。因此，为了避免这种情况，并且由于您将不得不迭代注释，所以您可以通过观察一个副作用来“检测”当注释为空时，如果comments为空，i将为零：

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"