SQLAlchemy：具有单个 Table 继承的邻接列表

Question

我正在尝试设置以下单table继承邻接表结构，其中子对象包含与父对象的关系：

class Parent(Base):
    __tablename__ = 'testtable'
    type = Column(String)
    __mapper_args__ = {
        'polymorphic_on':type,
        'polymorphic_identity':'Parent'
    }

    id = Column(Integer, primary_key = True)

class Child(Parent):
    __mapper_args__ = {
        'polymorphic_identity':'Child'
    }

    parent_id = Column(Integer, ForeignKey('testtable.id'))
    parent = relationship('Parent')

但是，当我尝试创建一个 Child 类型的对象并设置/附加一个父对象时：

p = Parent()
c = Child()
c.parent.append(p)

db_session.add(c)
db_session.commit()

我收到以下错误：

UnmappedColumnError：无法为目标列执行同步规则'testtable.parent_id'； mapper 'mapped class Parent->testtable' 不映射此列。尝试使用不包含此列的显式 foreign_keys 集合（或使用 viewonly=True 关系）。

我尝试过不同的关系定义（backref 等），但在这个阶段我很迷茫。任何想法将不胜感激！

非常感谢

为方便起见，这里有一个完整的例子：

from sqlalchemy import create_engine
from sqlalchemy.orm import sessionmaker
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy import Column, Integer, String
from sqlalchemy import ForeignKey
from sqlalchemy.orm import relationship

Base = declarative_base()
Session = sessionmaker()

engine = create_engine('sqlite:///test.db')
Session.configure(bind = engine)
db_session = Session()

class Parent(Base):
    __tablename__ = 'testtable'
    type = Column(String)
    __mapper_args__ = {
        'polymorphic_on':type,
        'polymorphic_identity':'Parent'
    }

    id = Column(Integer, primary_key = True)

class Child(Parent):
    __mapper_args__ = {
        'polymorphic_identity':'Child'
    }

    parent_id = Column(Integer, ForeignKey(Parent.id))
    parent = relationship('Parent', remote_side=[parent_id])

Base.metadata.create_all(engine)

p = Parent()
c = Child()
c.parent.append(p)

db_session.add(c)
db_session.commit()

Answer 1

您的 Child 对象有一个指向 Parent 的外键，因此它是一个多对一的关系。因此，您需要使用 c.parent = p 而不是 c.parent.append(p)。您还需要使用 remote_side=Parent.id 来建立 relationship.

工作示例：

from sqlalchemy import create_engine, Column, Integer, String, ForeignKey
from sqlalchemy.orm import declarative_base, relationship, Session

Base = declarative_base()

engine = create_engine("sqlite://")


class Parent(Base):
    __tablename__ = "testtable"
    type = Column(String)
    __mapper_args__ = {
        "polymorphic_on": type,
        "polymorphic_identity": "Parent",
    }

    id = Column(Integer, primary_key=True)

    def __repr__(self):
        return f"<Parent(id={self.id})>"


class Child(Parent):
    __mapper_args__ = {"polymorphic_identity": "Child"}

    parent_id = Column(Integer, ForeignKey(Parent.id))
    parent = relationship("Parent", remote_side=Parent.id)

    def __repr__(self):
        return f"<Child(id={self.id}, parent_id={self.parent_id})>"


Base.metadata.create_all(engine)

with Session(engine) as db_session:
    p = Parent()
    c = Child()
    c.parent = p
    db_session.add(c)
    db_session.commit()
    print(c)  # <Child(id=2, parent_id=1)>

SQLAlchemy：具有单个 Table 继承的邻接列表

SQLAlchemy: Adjacency List with Single Table Inheritance

python

sqlalchemy