On Sep 1, 2010, at 5:15 AM, Freewind wrote:
> I've two classes: Question and Answer. A question may have 0 or many
> answers.
>
> class Question(Base):
> __tablename__ = "questions"
> answers = relationship('Answer', backref='question',
>
> primaryjoin="Question.id==Answer.question_id")
>
> class Answer(Base):
> __tablename__ = "answers"
> Now I want to find all the questions have no answers, how to do it?
>
> I tried:
>
> Session.query(Question).filter('count(Question.answers)==0').all()
> It is incorrect. What is the right one?
In SQL, finding all the X with no related Y is always accomplished using a NOT
EXISTS predicate. SQLAlchemy provides an exists() construct in the general
case, and with a relationship() you can also use the any() operator which is a
more concise way to express it. See the tutorial at
http://www.sqlalchemy.org/docs/ormtutorial.html#using-exists (in this case
you'd want to say ~Question.answers.any()).
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