Question

我努力写这个问题。它进入了一个复杂而不常见的用例。

我在一个项目中定义了几个ORM类，负责维护公共数据库模式和核心功能。例如，让我们说这是model.email模块。

from sqlalchemy import Column, Index, ForeignKey
from sqlalchemy import Boolean, Integer, Text
from . import Base


class CampaignDB(Base):
    """
    Note: this basic database mapper class is expected to be extended.
    When sub-classing, be mindful to override mappings to other extended classes.
    """   
    __tablename__ = 'campaigns'

    audience_id = Column(Integer, ForeignKey("audiences.id"))
    active = Column(Boolean)
    name = Column(Text)

这些ORM类作为包导入到其他几个项目中。在某些情况下，这些ORM类被子类化以提供附加功能。例如，此处CampaignDB类是子类，以便为在特定项目上下文中发送电子邮件提供支持。

from model.email import CampaignDB

class Campaign(CampaignDB):
    """
    Provides sending capability to the email campaign ORM class.
    """

    def __init__(self, audience_id=None, active=None, name=None):
        self.audience_id = audience_id
        self.active = active
        self.name = name

    def send(self):
        print("send emails to the audience")

现在我想使用SQLAlchemy的类继承层次结构将CampaignDB和子类Campaign类重构为多态基。例如，我希望将CampaignDB作为EmailCampaignDB和PushCampaignDB的基类。然后，我想在导入项目中分别EmailCampaignDB和PushCampaignDB，EmailCampaign和PushCampaign。不过，我仍希望能够查询Campaign并返回EmailCampaign和PushCampaign的实例。

我已经多次尝试解决这个问题，但遇到了问题。特别是，session.query(Campaign).all()没有返回任何结果，因为SQLAlchemy似乎并不认为它是一个基类。生成的SQL具有以下WHERE子句：WHERE email.campaigns.type IN (NULL)

这是我正在尝试的主旨。

class CampaignDB(Base):
    """
    Note: this basic database mapper class is expected to be extended.
    When sub-classing, be mindful to override mappings to other extended classes.
    """
    __tablename__ = 'campaigns'

    audience_id = Column(Integer, ForeignKey("audiences.id"))
    active = Column(Boolean)
    name = Column(Text)
    type = Column(String(16))

    __mapper_args__ = {
        'polymorphic_on': type
    }


class EmailCampaignDB(CampaignBaseDB):
    __mapper_args__ = {
        'polymorphic_identity': 'email'
    }


class PushCampaignDB(CampaignBaseDB):
    __mapper_args__ = {
        'polymorphic_identity': 'push'
    }

    def send(self):
        print("send push notifications to the audience")

class Campaign(CampaignDB):

    pass

class EmailCampaign(EmailCampaignDB):

    def send(self):
        print("send emails to the audience")

class PushCampaign(PushCampaignDB):

    def send(self):
        print("send push notifications to the audience")

这可能吗？有没有更好的方法来实现这个＆＃34;打包的ORM＆＃34;上下文

Answer 1

我设法通过改变我对问题的看法来找到一种方法来完成这项工作。我放弃了尝试创建和查询Campaign的{{1}}子类。我还使用了Declarative API，它似乎有助于对CampaignDB和EmailCampaignDB进行子类化。

核心项目中的PushCampaignDB模块：

model.email

在导入项目中对广告系列类进行子类化：

from sqlalchemy.ext.declarative import declared_attr
from sqlalchemy import Column, Index, ForeignKey
from sqlalchemy.orm import relationship
from sqlalchemy import Boolean, Integer, Text
from . import Base

class CampaignBaseDB(Base):
    """
    Note: this basic database mapper class is expected to be extended.
    When sub-classing, be mindful to override mappings to other extended classes.
    """
    __tablename__ = 'campaign_bases'
    @declared_attr
    def __mapper_args__(cls):
        return {
            'polymorphic_on': cls.type,
        }

    audience_id = Column(Integer, ForeignKey("audiences.id"))
    active = Column(Boolean)
    name = Column(Text)
    type = Column(String(16))


class EmailCampaignDB(CampaignBaseDB):

    @declared_attr
    def __mapper_args__(cls):
        return {
            'polymorphic_identity': 'email'
        }


class PushCampaignDB(CampaignBaseDB):

    @declared_attr
    def __mapper_args__(cls):
        return {
            'polymorphic_identity': 'push'
        }

多态查询：

from model.email import EmailCampaignDB, PushCampaignDB

class EmailCampaign(EmailCampaignDB):

    def send(self):
        print("send emails to the audience")


class PushCampaign(PushCampaignDB):

    def send(self):
        print("send push notifications to the audience")

这种方法确实会导致＆＃34; SAWarning：重新分配身份的多态关联＆＃34;，所以我仍然觉得会有更好的方法。

从SQLAlchemy

1 个答案: