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在下面的代码中，我想all_holdings用一个名为holdings返回的属性替换 Account desired_holdings（这是代表最新已知数量的持有量，可以随时间变化）。我无法弄清楚如何构建对关系的调用。

此外，我希望对模式的适当性（将历史数据保存在单个表中并使用最大日期子查询获取最新数据）以及更好的替代方案或查询改进提出任何评论。

from sqlalchemy import Column, Integer, String, Date, DateTime, REAL, ForeignKey, func
from sqlalchemy.orm import relationship, aliased
from sqlalchemy.sql.operators import and_, eq
from sqlalchemy.ext.declarative import declarative_base
from db import session
import datetime
import string

Base = declarative_base()

class MySQLSettings(object):
    __table_args__ = {'mysql_engine':'InnoDB'}

class Account(MySQLSettings, Base):
    __tablename__ = 'account'
    id = Column(Integer, primary_key=True)
    name = Column(String(64))
    all_holdings = relationship('Holding', backref='account')

    def desired_holdings(self):
        max_date_subq = session.query(Holding.account_id.label('account_id'),
                                      Holding.stock_id.label('stock_id'),
                                      func.max(Holding.as_of).label('max_as_of')). \
                                      group_by(Holding.account_id, Holding.stock_id).subquery()

        desired_query = session.query(Holding).join(Account,
                                                    Account.id==account.id).join(max_date_subq).\
                                                    filter(max_date_subq.c.account_id==account.id).\
                                                    filter(Holding.as_of==max_date_subq.c.max_as_of).\
                                                    filter(Holding.account_id==max_date_subq.c.account_id).\
                                                    filter(Holding.stock_id==max_date_subq.c.stock_id)

        return desired_query.all()

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

class Stock(MySQLSettings, Base):
    __tablename__ = 'stock'
    id = Column(Integer, primary_key=True)
    name = Column(String(64))

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

class Holding(MySQLSettings, Base):
    __tablename__ = 'holding'
    id = Column(Integer, primary_key=True)
    account_id = Column(Integer, ForeignKey('account.id'), nullable=False)
    stock_id = Column(Integer, ForeignKey('stock.id'), nullable=False)
    quantity = Column(REAL)
    as_of = Column(Date)

    stock = relationship('Stock')

    def __str__(self):
        return "Holding(%f, '%s' '%s')"%(self.quantity, self.stock.name, str(self.as_of))

    def __init__(self, account, stock, quantity, as_of):
        self.account_id = account.id
        self.stock_id = stock.id
        self.quantity = quantity
        self.as_of = as_of

if __name__ == "__main__":
    ibm = Stock('ibm')
    session.add(ibm)
    account = Account('a')
    session.add(account)
    session.flush()
    session.add_all([ Holding(account, ibm, 100, datetime.date(2001, 1, 1)),
                      Holding(account, ibm, 200, datetime.date(2001, 1, 3)),
                      Holding(account, ibm, 300, datetime.date(2001, 1, 5)) ])
    session.commit()

    print "All holdings by relation:\n\t", \
        string.join([ str(h) for h in account.all_holdings ], "\n\t")

    print "Desired holdings query:\n\t", \
        string.join([ str(h) for h in account.desired_holdings() ], "\n\t")

运行时的结果是：

All holdings by relation:
    Holding(100.000000, 'ibm' '2001-01-01')
    Holding(200.000000, 'ibm' '2001-01-03')
    Holding(300.000000, 'ibm' '2001-01-05')
Desired holdings query:
    Holding(300.000000, 'ibm' '2001-01-05')
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1 回答 1

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在我发布到 sqlalchemy google 组后，Michael Bayer 提供了以下答案：

desired_holdings() 查询非常复杂，我没有看到通过尝试让 relationship() 来实现它的胜利。relationship() 旨在维护两个类之间的持久性，而不是报告技术（其中任何带有 max()/group_by 的东西都是指报告）。

我会将@property 放在desired_holdings 之上，使用object_session(self) 进入“会话”，然后完成。

查看有关启用查询的属性的更多信息。

于 2012-05-01T18:40:05.787 回答