SQLAlchemy func.count with filter

问题

I'm using a framework that does pagination like this:

def get_count_query(self):
    return self.session.query(func.count('*')).select_from(self.model)

def paginate(self):
    ... <irrelevant>...
    count = self.get_count_query.scalar()
    ...

I want to override the get_count_query method to use my own query because I'm filtering some results and get_count_query just returns all elements in the table. Queries are created dynamically, for example one query could be:

Asset.query.join(StatusLabel).filter(StatusLabel.status == 'Deployable', or_(
                                     Asset.assigned_to.isnot(None)),
                                     Asset.deleted_at.is_(None))

I can count the elements in this query easily with query.count():

def get_count_query(self):
    q = Asset.query.join(StatusLabel).filter(StatusLabel.status == 'Deployable', or_(
                                             Asset.assigned_to.isnot(None)),
                                             Asset.deleted_at.is_(None))
    return q.count()

But this will fail once it reach the .scalar() method (and I cannot remove this method). So the question is: how can I apply func.count('*') to an existing query?

Can I retrieve the filters from my query and apply them to the func.count('*') query?

回答1:

you can use select_from with join and filter

def get_count_query(self):
    return self.session.query(func.count('*')).select_from(Asset).join(StatusLabel)\
                                              .filter(StatusLabel.status == 'Deployable', or_(
                                                      Asset.assigned_to.isnot(None),
                                                      Asset.deleted_at.is_(None)))

with subquery

def get_count_query(self):
    q = Asset.query.join(StatusLabel).filter(StatusLabel.status == 'Deployable', or_(
                                             Asset.assigned_to.isnot(None)),
                                             Asset.deleted_at.is_(None))

    return self.session.query(func.count('*')).select_from(q.subquery())

来源：https://stackoverflow.com/questions/34446725/sqlalchemy-func-count-with-filter