基于Mixin列的SQLalchemy连接继承查询

Question

我有一个声明性的类结构，如下所示：

class BaseClass(Base):
      Column A
      Column B

class Mixin(object):
      Column C

class ItemA(BaseClass):
      Column D

class ItemB(Mixin, BaseClass):
      pass

class ItemC(Mixin, BaseClass):
      Column E

有没有一种方法可以with_polymorphic使我基于Column C所有对象进行查询Items，而不必明确知道自己Item拥有什么？本质上有点像

mixin_poly = with_polymorphic(base = BaseClass, classes = MixinClass.__subclasses__())

假设在导入时，在声明MixinClass之前，所有的导数都已导入with_polymorphic。

编辑 注意，我已经省略了连接表继承的样板，但是假设它已经正确设置，可以执行

poly_base = with_polymorphic(base = BaseClass, classes = '*')
session.query(poly_base).filter(BaseClass.a == value, ...)

执行预期的查询列BaseClass。关键是能够查询Mixin以相同方式继承该类的子类中的公共列。

Answer 1

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

Base = declarative_base()

class BaseClass(Base):
    __tablename__ = 'base'

    id = Column(Integer, primary_key=True)
    type = Column(String)
    a = Column(Integer)
    b = Column(Integer)
    __mapper_args__ = {"polymorphic_on": type}

class Mixin(object):
    c = Column(Integer)

class ItemA(BaseClass):
    __tablename__ = 'a'
    id = Column(ForeignKey('base.id'), primary_key=True)
    d = Column(Integer)
    __mapper_args__ = {"polymorphic_identity": 'a'}

class ItemB(Mixin, BaseClass):
    __tablename__ = 'b'
    id = Column(ForeignKey('base.id'), primary_key=True)
    __mapper_args__ = {"polymorphic_identity": 'b'}

class ItemC(Mixin, BaseClass):
    __tablename__ = 'c'
    id = Column(ForeignKey('base.id'), primary_key=True)
    e = Column(Integer)
    __mapper_args__ = {"polymorphic_identity": 'c'}

e = create_engine("sqlite://", echo=True)
Base.metadata.create_all(e)

def magic_w_poly(base):
    insp = inspect(base)

    w_poly = []
    crit = []

    for mapper in insp.self_and_descendants:
        if "c" in mapper.c:
            w_poly.append(mapper)
            crit.append(mapper.c.c)
    w_col_c = with_polymorphic(base, w_poly)

    def comparator(value):
        return or_(
                    crit_elem == value
                    for crit_elem in crit
                )

    return w_col_c, comparator

s = Session(e)

w_col, comp = magic_w_poly(BaseClass)

print s.query(w_col).filter(comp(35))

最后查询：

SELECT base.id AS base_id, base.type AS base_type, base.a AS base_a, base.b AS base_b, b.id AS b_id, b.c AS b_c, c.id AS c_id, c.c AS c_c, c.e AS c_e 
FROM base LEFT OUTER JOIN b ON base.id = b.id LEFT OUTER JOIN c ON base.id = c.id 
WHERE b.c = :c_1 OR c.c = :c_2