SqlAlchemy 与 dogpile 缓存

Question

我有三个模型，它们之间有继承和关系，我想把这些模型的查询结果缓存起来。

class Person(Base):
    __tablename__ = 'person'
    id = Column(Integer, primary_key=True)
    name = Column(String(100), nullable=False)
    type = Column(String(50))
    __mapper_args__ = {
        'polymorphic_identity': 'object',
        'polymorphic_on': type
        }

class Man(Person):
    __tablename__ = 'man'
    id = Column(Integer, ForeignKey('person.id'), primary_key=True)
    age = Column(String(100), nullable=False)
    __mapper_args__ = {'polymorphic_identity': 'man'}

class Config(Base):
    __tablename__ = "config"
    id = Column(Integer, primary_key=True)
    person = Column(Integer, ForeignKey('person.id'))
    address = Column(String)
    person_ref = relationship(Person)

还有很多其他模型是从Personal继承而来的。比如，我需要通过Config的关系来访问Man的属性。通常我会这样做：

config = session.query(Config).join(Config.person_ref).filter(Person.type == 'man').first()
print config.person_ref.age

我怎么才能用dogpile来缓存这样的查询呢？我可以缓存对Config的查询，但每次查询Man的属性时，都会发出SQL请求，无法缓存。我尝试使用with_polymorphic，但它只在没有joinedload的情况下有效。（我不明白为什么）

config = session.query(Config).options(FromCache("default")).first()
people = session.query(Person).options(FromCache("default")).with_polymorphic('*').get(config.person)

但我需要joinedload来进行类型过滤。

Answer 1

为了确保“man”表能够被加载，可以使用of_type()来处理任何子类型的模式。我们也可以使用with_polymorphic()来连接一个完整的多态选择。有关详细信息，请查看这个链接：http://docs.sqlalchemy.org/en/latest/orm/inheritance.html#creating-joins-to-specific-subtypes。只要你想要的数据能够通过一个SELECT查询出来，那么这些数据就会被缓存到FromCache中。确实，目前的缓存方法并没有包括一个系统，能够在之后缓存额外连接的继承属性的延迟加载。

from sqlalchemy import *
from sqlalchemy.orm import *
from sqlalchemy.ext.declarative import declarative_base
from examples.dogpile_caching.caching_query import query_callable, FromCache, RelationshipCache
from hashlib import md5
from dogpile.cache.region import make_region

Base = declarative_base()

class Person(Base):
    __tablename__ = 'person'
    id = Column(Integer, primary_key=True)
    name = Column(String(100), nullable=False)
    type = Column(String(50))
    __mapper_args__ = {
        'polymorphic_identity': 'object',
        'polymorphic_on': type
        }

class Man(Person):
    __tablename__ = 'man'
    id = Column(Integer, ForeignKey('person.id'), primary_key=True)
    age = Column(String(100), nullable=False)
    __mapper_args__ = {'polymorphic_identity': 'man'}

class SomethingElse(Person):
    __tablename__ = 'somethingelse'

    id = Column(Integer, ForeignKey('person.id'), primary_key=True)
    age = Column(String(100), nullable=False)
    __mapper_args__ = {'polymorphic_identity': 'somethingelse'}

class Config(Base):
    __tablename__ = "config"
    id = Column(Integer, primary_key=True)
    person = Column(Integer, ForeignKey('person.id'))
    address = Column(String)
    person_ref = relationship(Person)

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

def md5_key_mangler(key):
    """Receive cache keys as long concatenated strings;
    distill them into an md5 hash.

    """
    return md5(key.encode('ascii')).hexdigest()

regions = {}
regions['default'] = make_region(
            key_mangler=md5_key_mangler
            ).configure(
                'dogpile.cache.memory_pickle',
            )

Session = scoped_session(
                sessionmaker(
                    bind=e,
                    query_cls=query_callable(regions)
                )
            )

sess = Session()
sess.add(Config(person_ref=SomethingElse(age='45', name='se1')))
sess.add(Config(person_ref=Man(age='30', name='man1')))
sess.commit()

all_types = with_polymorphic(Person, "*", aliased=True)

conf = sess.query(Config).options(joinedload(Config.person_ref.of_type(all_types)), FromCache("default")).first()
sess.commit()
sess.close()

print "_____NO MORE SQL!___________"


conf = sess.query(Config).options(joinedload(Config.person_ref.of_type(all_types)), FromCache("default")).first()
print conf.person_ref.age