Count on related model as hybrid property

问题

I have a table called SectionAttempt which has many QuestionAttempts

I want to display a computed field 'total_count' that is the number of question attempts for the SectionAttempt WITHOUT loading all the questionAttempts.

Looking online , I found this:

select([func.count()],
QuestionAttempt.__table__.c.section_attempt_id==SectionAttempt.__table__.c.id)
.correlate(SectionAttempt.__table__)
.as_scalar()

I am not 100% sure why this works, but it is working when I query like this:

 test_attempt = (SOMEQUERY
                .undefer(SectionAttempt.question_attempt_total_count)).one())

I have questions:

1. what is .correlate doing here.
2. The way it is added to the model, makes me have to import my QuestionAttempt model, which is prone to cyclic import issues (what if I have to do something similar on my QuestionAttempt model ). How can I retain the functionality but either change to a string based reference (like we do for relationships), or inside a function (so that I can import Question Attempt inside the function instead of globally)

来源：https://stackoverflow.com/questions/54168059/count-on-related-model-as-hybrid-property