SQLAlchemy - 为子计数编写混合方法 [英] SQLAlchemy - Writing a hybrid method for child count
问题描述
我正在使用 Flask-SQLAlchemy,我正在尝试编写一个 混合方法 在父模型中返回它所拥有的孩子的数量,所以我可以用它来过滤、排序等.这是我正在尝试的一些精简代码:
I'm using Flask-SQLAlchemy, and I'm trying to write a hybrid method in a parent model that returns the number of children it has, so I can use it for filtering, sorting, etc. Here's some stripped down code of what I'm trying:
# 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
我在这里遇到了两个问题.一方面,我不知道在child_count(cls)"中究竟要返回什么,它必须是一个 SQL 表达式......我认为它应该像
I'm running into two problems here. For one, I don't know what exactly to return in the "child_count(cls)", which has to be an SQL expression... I think it should be something like
return select([func.count('*'), from_obj=Child).where(Child.parent_id==cls.parent_id).label('Child count')
但我不确定.我遇到的另一个问题是我无法从 parent.py 导入 Child 类,因此无论如何我都无法使用该代码.有没有办法为此使用字符串?例如,
but I'm not sure. Another issue I have is that I can't import the Child class from parent.py, so I couldn't use that code anyway. Is there any way to use a string for this? For example,
select([func.count('*'), from_obj='children').where('children.parent_id==parents.parent_id').label('Child count')
最终,我想将方法更改为:
Eventually, I'll want to change the method to something like:
def child_count(cls, start_time, end_time):
# return the number of children whose "date" parameter is between start_time and end_time
...但就目前而言,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,因为我一直在努力解决这个问题.
...but for now, I'm just trying to get this to work. Huge thanks to whoever can help me with this, as I've been trying to figure this out for a long time now.
推荐答案
下面的代码说明了一切.
The code below shows it all.
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)
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