當在查詢中使用「group_by」時，從每個組中選擇一個元素

Question

我想知道如何在查詢「group by」時從類中提取任意元素。例如：

class Post(Base): 
    id = Column(Integer, primary_key=True) 
    created = Column(DateTime) 
    comments = Column(Integer) 
    creator = Column(Integer, ForeignKey('user.id')) 
    anonymous = Column(Boolean) 

class User(Base): 
    id = Column(Integer, primary_key=True) 

我將如何創建一個輸出的查詢：

creator | total number of comments made on the creator's post | the time of the creator's last post 

creator | total number of comments made on the creator's post | if any of the posts are anonymous 

基本的查詢是：

session = DBSession() 
commsum = session.query(Post.creator, func.sum(Post.comments).label('totalcomments')).\ 
    group_by(Post.creator).subquery() 
return session.query(User, commsum.c.totalcomments).\ 
    join(commsum, commsum.c.creator == User.id).all() 

如果我想回到1）的最新日期每個group_by（Post.creator）中的所有行或者2）如果任何行具有匿名== True。

Answer 1

在SQL中，你應該能夠做這樣的事情：

SELECT Post.creator, 
     sum(Post.comments) total_comments 
     max(Post.Created) latest_post 
     max(Post.anonymous) any_anonymous 
FROM Post 
GROUP BY Post.creator