如何检查GQL对象是否为空

Question

我想检查一个GQL对象是否为空（在使用Python的Google App Engine上），我该怎么做呢？我试过这个方法，但没有成功：

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

根据@NickJohnson的说法，调用count这个函数会让你执行同样的查询两次。为了避免这种情况，而且因为你需要逐个查看评论，所以不如通过观察一个副作用来“检测”评论是否为空——也就是如果comments是空的，i的值会是零：

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"

Answer 2

Gql查询会给你一个查询对象，但不会直接给你结果（也就是数据实体）。要获取结果，你需要通过一些方法来提取这些结果，比如使用fetch、count、get，或者遍历结果集。