SqlAlchemy 数据类行为和声明性混合：AttributeError：无法设置属性

Question

我有一组作为纯 python 数据类起草的类。这是一个精简的示例，其行为与我预期的一样：

from dataclasses import dataclass
from abc import ABC

@dataclass
class Owner:
    name: str = ""

@dataclass
class OwnedThing(ABC):
    owner: Owner = None

@dataclass
class Thing1(OwnedThing):
    name: str = ""


owner = Owner(name="foo")
thing = Thing1( name="bar", owner=owner)
print(thing.owner.name)

这里我们有一个基类，它确定许多派生类都将具有一个“所有者”对象。分配所有者很简单。

当我尝试添加 SqlAlchemy 以实现持久性时，我想保留一般继承结构，这样我就不必为所有各种拥有的事物重复这种所有者-事物关系。但是，当我这样做时，我不能再将所有者类实例分配给 .owner 属性：

from dataclasses import dataclass,field
from abc import ABC
from sqlalchemy.orm import declarative_mixin, declared_attr, relationship
from sqlalchemy import Column, ForeignKey

@dataclass
class Owner:
    name: str = ""

@declarative_mixin
@dataclass
class OwnedThing(ABC):
    owner_id: int = field(init=False, default=None)
    owner: Owner = None

    @declared_attr
    def owner_id(cls):
        return Column('owner_id', ForeignKey('owner.id'))

    @declared_attr
    def owner(cls):
        return relationship(Owner)

@dataclass
class Thing1(OwnedThing):
    name: str = ""


owner = Owner(name="foo")
thing = Thing1( name="bar", owner=owner)
print(thing.owner.name)

这将导致此堆栈跟踪：

/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/dataclasses.py:658: SAWarning: Unmanaged access of declarative attribute owner_id from non-mapped class OwnedThing
  default = getattr(cls, a_name, MISSING)
/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/dataclasses.py:658: SAWarning: Unmanaged access of declarative attribute owner from non-mapped class OwnedThing
  default = getattr(cls, a_name, MISSING)
/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/dataclasses.py:842: SAWarning: Unmanaged access of declarative attribute owner_id from non-mapped class OwnedThing
  if isinstance(getattr(cls, f.name, None), Field):
/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/dataclasses.py:842: SAWarning: Unmanaged access of declarative attribute owner from non-mapped class OwnedThing
  if isinstance(getattr(cls, f.name, None), Field):
Traceback (most recent call last):
  File "test_case_sql.py", line 30, in <module>
    thing = Thing1( name="bar", owner=owner)
  File "<string>", line 2, in __init__
AttributeError: can't set attribute

请注意，当我跳过基类时，事情会按我的预期工作，没有错误：

from dataclasses import dataclass,field
from abc import ABC
from sqlalchemy.orm import declarative_mixin, declared_attr, relationship
from sqlalchemy import Column, ForeignKey,Integer

@dataclass
class Owner:
    name: str = ""


@dataclass
class Thing1:
    owner_id: int = Column(Integer)
    owner: Owner = relationship("Owner")
    name: str = ""


owner = Owner(name="foo")
thing = Thing1( name="bar", owner=owner)
print(thing.owner.name)

我对 SqlAlchemy 的内部结构了解得不够多，无法理解我做错了什么，而且我之前也没有看到其他人遇到过这个问题。有人可以帮我继续使用数据类和混合吗？

Answer 1

事实证明，在类周围添加映射代码可以解决问题：

from dataclasses import dataclass,field
from abc import ABC
from sqlalchemy.orm import declarative_mixin, declared_attr, relationship, registry
from sqlalchemy import Column, ForeignKey, Integer

mapper_registry = registry()

@mapper_registry.mapped
@dataclass
class Owner:
    __tablename__ = "owner"
    id: int = Column(Integer, primary_key=True)

    name: str = ""

@declarative_mixin
@dataclass
class OwnedThing(ABC):
    id: int = Column(Integer, primary_key=True)
    owner_id: int = field(init=False, default=None)
    owner: Owner = None

    @declared_attr
    def owner_id(cls):
        return Column('owner_id', ForeignKey('owner.id'))

    @declared_attr
    def owner(cls):
        return relationship(Owner)

@mapper_registry.mapped
@dataclass
class Thing1(OwnedThing):
    __tablename__ = "thing"

    name: str = ""


owner = Owner(name="foo")
thing = Thing1( name="bar", owner=owner)
print(thing.owner.name)

仍有一些警告，但代码现在按预期运行。