python - 无法在单表继承设置中的子类上定义 __table_args__

Question

我发现如果 __table_args__ 存在于单表继承设置中的子类中，SQLAlchemy 会抛出 sqlalchemy.exc.ArgumentError: Can't place __table_args__ on an inherited没有表的类。同时，可以在子类上定义一个带有 index=True 的列，这会像 __table_args__ 一样改变父表。

这是我的设置:

import sqlalchemy as sa
import sqlalchemy.ext.declarative

from sqlalchemy.ext.declarative import declarative_base


Base = declarative_base()


class A(Base):
    __tablename__ = 'a'

    id = sa.Column(sa.Integer, primary_key=True)
    type = sa.Column(sa.Text, nullable=False)

    __mapper_args__ = {
        'polymorphic_on': type,
    }


class B(A):
    b = sa.Column(sa.Integer, index=True)

    __mapper_args__ = {
        'polymorphic_identity': 'b',
    }


class C(A):
    c = sa.Column(sa.Integer)

    __mapper_args__ = {
        'polymorphic_identity': 'c',
    }

    __table_args__ = (
        sa.Index('ix_test', c),
    )


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

session.add_all([
    B()
])

session.commit()

print(session.query(A))

它抛出:

Traceback (most recent call last):
 File "test.py", line 29, in <module>
   class C(A):
 File "/home/andrei/projects/my_project/.tox/dev/lib/python3.5/site-packages/sqlalchemy/ext/decla
rative/api.py", line 64, in __init__
   _as_declarative(cls, classname, cls.__dict__)
 File "/home/andrei/projects/my_project/.tox/dev/lib/python3.5/site-packages/sqlalchemy/ext/decla
rative/base.py", line 88, in _as_declarative
   _MapperConfig.setup_mapping(cls, classname, dict_)
 File "/home/andrei/projects/my_project/.tox/dev/lib/python3.5/site-packages/sqlalchemy/ext/decla
rative/base.py", line 103, in setup_mapping
   cfg_cls(cls_, classname, dict_)
 File "/home/andrei/projects/my_project/.tox/dev/lib/python3.5/site-packages/sqlalchemy/ext/decla
rative/base.py", line 133, in __init__
   self._setup_inheritance()
 File "/home/andrei/projects/my_project/.tox/dev/lib/python3.5/site-packages/sqlalchemy/ext/decla
rative/base.py", line 442, in _setup_inheritance
   "Can't place __table_args__ on an inherited class "
sqlalchemy.exc.ArgumentError: Can't place __table_args__ on an inherited class with no table.

有人知道任何解决方法吗？

Answer 1

索引可以放在表定义之外。

class C(A):
    c = sa.Column(sa.Integer)

    __mapper_args__ = {
        'polymorphic_identity': 'c',
    }

Index('ix_test', C.c)