当不存在行时 sqlalchemy func.sum() 返回 None

Question

我成功地获得了一个查询，该查询总结了父类的所有子行：

subq = db.session.query(Transaction.budget_id, func.sum(Transaction.amount).label("total"), func.count('*').label("count"))
  .group_by(Transaction.budget_id).subquery()

return db.session.query(Budget.ID, Budget.name, Budget.start_date, Budget.end_date, Budget.amount, subq.c.total, subq.c.count)
  .outerjoin(subq, Budget.ID==subq.c.budget_id)
  .order_by(Budget.start_date.desc()).limit(count)

问题是当预算没有任何交易子类时它不起作用。它返回 None，这给工作带来了麻烦。我希望 sum() 和 count() 函数返回 0。

Answer 1

from sqlalchemy.sql.functions import coalesce

...

return db.session.query(Budget.ID, 
                        Budget.name, 
                        Budget.start_date,
                        Budget.end_date,
                        Budget.amount,
                        coalesce(subq.c.total, 0),
                        coalesce(subq.c.count, 0))
      .outerjoin(subq, Budget.ID==subq.c.budget_id)
      .order_by(Budget.start_date.desc()).limit(count)