SQLAlchemy多态多对多关系与声明性语法中的抽象基

Question

我正在创建一个具有多对多关系的 SQLAlchemy 声明性模型，其中一个表是具体的，另一个是具有抽象基础的多态表。

用例：我正在发送包含多个组件（component1、component2）的消息，并且每个组件都有非常不同的属性集（因此每个组件都有自己的数据库表）。一个组件可以在多个不同的消息中发送。

我想在 Message 类和一个名为 Component 的抽象父类之间建立 M:N 关系 - 但数据库不应包含任何抽象的“组件”表，只有具体的表子项（“component1”、“component2”等）。

我尝试将abstract 1:N relation using AbstractConcreteBase（参见本章最后/第三个代码sn-p）与regular M:N relation组合成以下代码，但无法找到抽象基类的表名：

from sqlalchemy import *
from sqlalchemy.orm import *
from sqlalchemy.ext.declarative import declarative_base, AbstractConcreteBase

BaseModel = declarative_base()

association = Table("message_component_association", BaseModel.metadata,
    Column("message_id", Integer, ForeignKey("message.id")),
    Column("component_id", Integer, ForeignKey("component.id")))  # Fails to reference the Component class

class Message(BaseModel):
    __tablename__ = "message"
    id = Column(Integer, primary_key=True)
    components = relationship("Component", secondary=association, back_populates="messages")

class Component(AbstractConcreteBase, BaseModel):
    __mapper_args__ = dict(polymorphic_identity="component", concrete=False)  # This seems to be ignored

class Component1(Component):
    __tablename__ = "component1"
    __mapper_args__ = dict(polymorphic_identity="component1", concrete=True)
    id = Column(Integer, primary_key=True)
    messages = relationship("Message", secondary=association, back_populates="components")

engine = create_engine("sqlite://")
BaseModel.metadata.create_all(engine)

异常说：

sqlalchemy.exc.NoReferencedTableError: Foreign key associated with column 'message_component_association.component_id' could not find table 'component' with which to generate a foreign key to target column 'id'

为什么 Component 类的 ma​​pper_args 被忽略，并且通过提供的 polymorphic_identity 找不到该类？

编辑：我意识到我可以使用 Joined Table Inheritance（grr，我不能发布超过 2 个链接），它用显式鉴别器替换声明性帮助器 mixin 以获得多态 M:N 关系- 但是，基类需要自己的数据库表。

from sqlalchemy import *
from sqlalchemy.orm import *
from sqlalchemy.ext.declarative import declarative_base

BaseModel = declarative_base()

association = Table("message_component_association", BaseModel.metadata,
Column("message_id", Integer, ForeignKey("message.id")),
Column("component_id", Integer, ForeignKey("component.id")))

class Message(BaseModel):
    __tablename__ = "message"
    id = Column(Integer, primary_key=True)
    components = relationship("Component", secondary=association, back_populates="messages")

class Component(BaseModel):  # Declarative mixin removed
    __tablename__ = "component"  # Requires a real DB table despite being abstract
    __mapper_args__ = dict(polymorphic_identity="component", polymorphic_on="type")  # Apply the discriminator
    id = Column(Integer, primary_key=True)
    type = Column(String(32))  # Explicit discriminator
    messages = relationship("Message", secondary=association, back_populates="components")

class Component1(Component):
    __tablename__ = "component1"
    __mapper_args__ = dict(polymorphic_identity="component1")
    id = Column(Integer, ForeignKey("component.id"), primary_key=True)  # Shares the primary key sequence with the parent and with all other child classes
    messages = relationship("Message", secondary=association, back_populates="components")

engine = create_engine("sqlite://", echo=True)
BaseModel.metadata.create_all(engine)
session = Session(engine)

component_1 = Component1(id=1)
session.commit()

到目前为止，该代码似乎可以正常工作，但它抱怨刷新问题。只要我不手动写入“组件表”，忽略警告是否安全 - 还是有更好的方法？

SAWarning: Warning: relationship 'messages' on mapper 'Mapper|Component1|component1' supersedes the same relationship on inherited mapper 'Mapper|Component|component'; this can cause dependency issues during flush

Answer 1

解决方案： 删除除 Message 类中的关系之外的所有关系，并将 back_populates 替换为 backref。 Backref 将动态创建相反的方向，并且映射器不会看到被覆盖的关系。此外，抽象祖先上的 polymorphic_identity 也不是必需的。

from sqlalchemy import *
from sqlalchemy.orm import *
from sqlalchemy.ext.declarative import declarative_base

BaseModel = declarative_base()

association = Table("message_component_association", BaseModel.metadata,
Column("message_id", Integer, ForeignKey("message.id")),
Column("component_id", Integer, ForeignKey("component.id")))

class Message(BaseModel):
    __tablename__ = "message"
    id = Column(Integer, primary_key=True)
    components = relationship("Component", secondary=association, backref="messages")  # backref instead of back_populates

class Component(BaseModel):
    __tablename__ = "component"
    __mapper_args__ = dict(polymorphic_on="type")  # Polymorphic identity removed
    id = Column(Integer, primary_key=True)
    type = Column(String(32))

    # relationship removed

class Component1(Component):
    __tablename__ = "component1"
    __mapper_args__ = dict(polymorphic_identity="component1")
    id = Column(Integer, ForeignKey("component.id"), primary_key=True)

    # relationship removed

engine = create_engine("sqlite://", echo=True)
BaseModel.metadata.create_all(engine)
session = Session(engine)

component_1 = Component1(id=1)
session.commit()