Question

我正在尝试检查GQL对象是否为空（在使用Python的Google App Engine中），我应该怎么做？我试过这个，但它不起作用：

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查询为您提供查询对象，但不提供结果（实体）。要获得结果，您必须获取结果：使用fetch，count，get或遍历结果集。

Answer 2

According to @NickJohnson，调用count需要执行两次相同的查询。所以为了避免这种情况，并且由于你将不得不迭代注释，你也可以通过观察副作用“检测”注释是否为空 - 如果{i为comments 1}}为空：

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"

如何检查GQL对象是否为空

2 个答案: