如何使用 sqlalchemy declarative base 多层次继承

在SQLAlchemy中，通过declarative_base创建的基类可以通过多层次的继承建立继承关系。这允许你在数据库中创建具有继承结构的表。在我使用某数据库做中转的时候，经常会遇到各种各样的问题，例如下面的问题，通过记录并附上完美的解决方案，以便为查询更加方便。

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问题背景

在使用 sqlalchemy declarative base 建立模型时，我有许多具有相同列的表。不同的只是表名。我想设置一个继承链来最小化代码重复。以下单层继承按我想要的方式工作：

from sqlalchemy import Column, Integer, Text
from sqlalchemy.ext.declarative import declarative_base, declared_attr
from sqlalchemy.orm import sessionmaker

engine = sqlalchemy.create_engine('sqlite:///monDom5.db')


class Base(object):
    """base for all table classes"""
    __abstract__ = True
    __table_args__ = {'autoload': True, 'autoload_with': engine}
    @declared_attr
    def __tablename__(cls):
        return cls.__name__.lower()


Base = declarative_base(cls=Base)


class TransMap_HgmIntronVector(Base):
    AlignmentId = Column(Text, primary_key=True)

但是需要为每个 Hgm 基类的实例指定 AlignmentId 列。相反，我想这样做，但在尝试实际使用它时出现 sqlalchemy.exc.InvalidRequestError 错误：

from sqlalchemy import Column, Integer, Text
from sqlalchemy.ext.declarative import declarative_base, declared_attr
from sqlalchemy.orm import sessionmaker

engine = sqlalchemy.create_engine('sqlite:///monDom5.db')

class Base(object):
    """base for all table classes"""
    __abstract__ = True
    __table_args__ = {'autoload': True, 'autoload_with': engine}
    @declared_attr
    def __tablename__(cls):
        return cls.__name__.lower()


# model for all Hgm tables

class Hgm(Base):
    __abstract__ = True
    AlignmentId = Column(Text, primary_key=True)


Base = declarative_base(cls=Hgm)

class TransMap_HgmIntronVector(Hgm):
    pass



metadata = Base.metadata
Session = sessionmaker(bind=engine)
session = Session()

导致错误

>>> metadata = Base.metadata
>>> Session = sessionmaker(bind=engine)
>>> session = Session()
>>> session.query(TransMap_HgmIntronVector).all()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/cluster/home/ifiddes/anaconda2/lib/python2.7/site-packages/sqlalchemy/orm/session.py", line 1260, in query
    return self._query_cls(entities, self, **kwargs)
  File "/cluster/home/ifiddes/anaconda2/lib/python2.7/site-packages/sqlalchemy/orm/query.py", line 110, in __init__
    self._set_entities(entities)
  File "/cluster/home/ifiddes/anaconda2/lib/python2.7/site-packages/sqlalchemy/orm/query.py", line 118, in _set_entities
    entity_wrapper(self, ent)
  File "/cluster/home/ifiddes/anaconda2/lib/python2.7/site-packages/sqlalchemy/orm/query.py", line 3829, in __init__
    "expected - got '%r'" % (column, )
sqlalchemy.exc.InvalidRequestError: SQL expression, column, or mapped entity expected - got '<class '__main__.TransMap_HgmIntronVector'>'

解决方案

官方文档中提供了一个示例。特别地，__abstract__ = True 是没有必要的。这样可以正常工作：

class Base(object):
    @declared_attr
    def __tablename__(cls):
        return cls.__name__.lower()

class Hgm(Base):
    AlignmentId = Column(Text, primary_key=True)

Base = declarative_base(cls=Hgm)

class TransMap_HgmIntronVector(Base):
    pass