SQLAlchemy - 为子计数编写混合方法

Question

我正在使用Flask-SQLAlchemy,我正在尝试在父模型中编写一个混合方法,它返回它拥有的子项数,因此我可以使用它来进行过滤,排序等.这里有一些简化的代码我尝试着:

# parent.py
from program.extensions import db
from sqlalchemy.ext.hybrid import hybrid_method

class Parent(db.Model):
    __tablename__ = 'parents'
    parent_id = db.Column(db.Integer, primary_key=True)

    name = db.Column(db.String(80))
    children = db.relationship('Child', backref='parent', lazy='dynamic')

    def __init__(self, name):
        self.name = name

    @hybrid_method
    def child_count(self):
        return self.children.count()

    @child_count.expression
    def child_count(cls):
        return ?????

# child.py
from program.extensions import db
from program.models import Parent

class Child(db.Model):
    __tablename__ = 'children'
    child_id = db.Column(db.Integer, primary_key=True)
    parent_id = db.Column(db.Integer, db.ForeignKey(Parent.parent_id))

    name = db.Column(db.String(80))
    time = db.Column(db.DateTime)

    def __init__(self, name, time):
        self.name = name
        self.time = time

我在这里遇到两个问题.首先,我不知道究竟要在"child_count(cls)"中返回什么,这必须是一个SQL表达式...我认为它应该像

return select([func.count('*'), from_obj=Child).where(Child.parent_id==cls.parent_id).label('Child count')

但我不确定.我的另一个问题是我无法从parent.py导入Child类,所以无论如何我都无法使用该代码.有没有办法使用字符串？例如,

select([func.count('*'), from_obj='children').where('children.parent_id==parents.parent_id').label('Child count')

最后,我想将方法​​更改为:

def child_count(cls, start_time, end_time):
    # return the number of children whose "date" parameter is between start_time and end_time

......但是现在,我只是想让它发挥作用.非常感谢能帮助我解决这个问题的人,因为我一直试图解决这个问题很长一段时间.

Answer 1

下面的代码显示了一切.

class Parent(Base):
    __tablename__ = 'parents'
    # ...

    @hybrid_property
    def child_count(self):
        #return len(self.children)   # @note: use when non-dynamic relationship
        return self.children.count()# @note: use when dynamic relationship

    @child_count.expression
    def child_count(cls):
        return (select([func.count(Child.child_id)]).
                where(Child.parent_id == cls.parent_id).
                label("child_count")
                )

    @hybrid_method
    def child_count_ex(self, stime, etime):
        return len([_child for _child in self.children
            if stime <= _child.time <= etime ])

    @child_count_ex.expression
    def child_count_ex(cls, stime, etime):
        return (select([func.count(Child.child_id)]).
                where(Child.parent_id == cls.parent_id).
                where(Child.time >= stime).
                where(Child.time <= etime).
                label("child_count")
                )


# usage of expressions:
stime, etime = datetime.datetime(2012, 1, 1), datetime.datetime(2012, 1, 31)
qry = session.query(Parent)
#qry = qry.filter(Parent.child_count > 2)
qry = qry.filter(Parent.child_count_ex(stime, etime) > 0)