How to update the user.question_count when the user posted a question?

I'm using sqlalchemy. The question is simple, but I have a big problem, because I want to use MapperExtension.

I have two classes: User and Question

A user may have many questions, and it also contains a question_count to record the the count of questions belong to him.

So, when I add a new question, I want update the question_count of the user. At first, I do as:

question = Question(title='aaa', content='bbb') 
Session.add(question) 
Session.flush() 


user = question.user 
### user is not None 
user.question_count += 1 
Session.commit() 

Everything goes well.

But I wan't to use event callback to do the same thing, so I use the MapperExtension of sqlalchemy. I put the logic of updating user.question_count in a after_insert callback of question. But I found I can't up开发者_运维知识库date the user.question_count, it will be ignored.

I don't know the reason. And, if you have the same requirement, how do you implement it?

The unit of work is computed before mapper extension is executed, so your changes to user object are left pending after transaction is committed first time. I suggest you not using ORM to update question_count, but execute SQL statement directly (use User.__table__.update(...) to generate statement).

Also there is a common idiom to atomically increment column: table.update(...).values(col=table.c.col+1) which is translated to UPDATE ... SET col=col+1, while your code just sets new value losing possible intermediate increments.