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from sqlalchemy.orm.query import Query, _generative
from sqlalchemy.orm.session import Session
# the cache. This would be replaced with the caching mechanism of
# choice, i.e. LRU cache, memcached, etc.
_cache = {}
class CachingQuery(Query):
# generative method to set a "cache" key. The method of "keying" the cache
# here can be made more sophisticated, such as caching based on the query._criterion.
@_generative()
def with_cache_key(self, cachekey):
self.cachekey = cachekey
# single point of object loading is __iter__(). objects in the cache are not associated
# with a session and are never returned directly; only merged copies.
def __iter__(self):
if hasattr(self, 'cachekey'):
try:
ret = _cache[self.cachekey]
except KeyError:
ret = list(Query.__iter__(self))
for x in ret:
self.session.expunge(x)
_cache[self.cachekey] = ret
return iter(self.session.merge(x, dont_load=True) for x in ret)
else:
return Query.__iter__(self)
# example usage
if __name__ == '__main__':
from sqlalchemy import Column, create_engine, Integer, String
from sqlalchemy.orm import sessionmaker
from sqlalchemy.ext.declarative import declarative_base
Session = sessionmaker(query_cls=CachingQuery)
Base = declarative_base(engine=create_engine('sqlite://', echo=True))
class User(Base):
__tablename__ = 'users'
id = Column(Integer, primary_key=True)
name = Column(String(100))
def __repr__(self):
return "User(name=%r)" % self.name
Base.metadata.create_all()
sess = Session()
sess.add_all(
[User(name='u1'), User(name='u2'), User(name='u3')]
)
sess.commit()
# cache two user objects
sess.query(User).with_cache_key('u2andu3').filter(User.name.in_(['u2', 'u3'])).all()
sess.close()
sess = Session()
# pull straight from cache
print sess.query(User).with_cache_key('u2andu3').all()
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