Thanks for the quick response!
I implemented your suggestion and I think that's perfectly fine for me, as
there's no use case for loading ConfigParam instances on their own.
cheers
Am Mittwoch, 8. April 2015 17:16:44 UTC+2 schrieb Michael Bayer:
>
>
>
> On 4/8/15 3:22 AM, Sebastian Eckweiler wrote:
>
> Hi there -
>
> I'm having trouble working with backrefs of detached objects.
> I'm basically working with a extended version of the code below:
>
> class Config(Base):
> __tablename__ = 'config'
>
> ID = Column('ID', Integer, primary_key=True)
> name = Column('name', String)
> last_modified = Column('last_modified', DateTime, default=now,
> onupdate=now)
>
> params = relationship('ConfigParam', backref='config', lazy=False)
>
> class ConfigParam(Base):
>
> __tablename__ = 'config_params'
>
> ID = Column('ID', Integer, primary_key=True)
> ConfigID = Column('ConfigID', Integer, ForeignKey('config.ID'),
> nullable=False)
>
> key = Column('key', String)
> value = Column('value', Float)
>
> Now when I load a Config instance I'd have assumed that
> Config.params[0].config should be populated - since the eager query
> contains all necessary information.
>
> A load that can be resolved to using the session's identity map instead of
> SELECT is still a load nonetheless. You would need a lazy setting on the
> backref as well. Since these are many-to-ones and the lazyload pulls from
> identity map, I suggest "immediate".
>
> Using these settings at the mapping level does mean however that if you
> load a ConfigParam object by itself, it will, for a clean session,
> immediately issue a second SELECT statement for its Config object.
>
>
>
>
> Testing this with a sqlite engine using the following code:
>
>
> s = Session()
> c = Config(name='my_config')
> c.params += [ConfigParam(key='a', value=1),
> ConfigParam(key='b', value=2)]
> s.add(c)
> s.commit()
> s.close()
> del c
>
> # reload from new session:
> logging.info('\n\n')
> logging.info('Starting with new session:')
> s = Session()
> c = s.query(Config).filter(Config.name=='my_config').one()
> s.close()
> for p in c.params:
> logging.info('Params:%s = %s' % (p.key, p.value))
> for p in c.params:
> logging.info('backref: config = %s' % p.config)
>
> however produces (omitting the create_all) the following output:
>
> INFO:sqlalchemy.engine.base.Engine:()
> INFO:sqlalchemy.engine.base.Engine:COMMIT
> INFO:sqlalchemy.engine.base.Engine:BEGIN (implicit)
> INFO:sqlalchemy.engine.base.Engine:INSERT INTO config (name,
> last_modified) VALUES (?, ?)
> INFO:sqlalchemy.engine.base.Engine:('my_config', '2015-04-08
> 09:04:16.111000')
> INFO:sqlalchemy.engine.base.Engine:INSERT INTO config_params ("ConfigID",
> "key", value) VALUES (?, ?, ?)
> INFO:sqlalchemy.engine.base.Engine:(1, 'a', 1.0)
> INFO:sqlalchemy.engine.base.Engine:INSERT INTO config_params ("ConfigID",
> "key", value) VALUES (?, ?, ?)
> INFO:sqlalchemy.engine.base.Engine:(1, 'b', 2.0)
> INFO:sqlalchemy.engine.base.Engine:COMMIT
> INFO:root:
>
>
> INFO:root:Starting with new session:
> INFO:sqlalchemy.engine.base.Engine:BEGIN (implicit)
> INFO:sqlalchemy.engine.base.Engine:SELECT config."ID" AS "config_ID",
> config.name AS config_name, config.last_modified AS config_last_modified,
> config_params_1."ID" AS "config_params_1_ID", config_params_1."ConfigID" AS
> "config_params_1_ConfigID", config_params_1."key" AS config_params_1_key,
> config_params_1.value AS config_params_1_value
> FROM config LEFT OUTER JOIN config_params AS config_params_1 ON
> config."ID" = config_params_1."ConfigID"
> WHERE config.name = ?
> INFO:sqlalchemy.engine.base.Engine:('my_config',)
> INFO:sqlalchemy.engine.base.Engine:ROLLBACK
> INFO:root:Params:a = 1.0
> INFO:root:Params:b = 2.0
> Traceback (most recent call last):
> File "D:/Assess/Kiln/assess/scratch/sqla_example.py", line 77, in
> <module>
> logging.info('backref: config = %s' % p.config)
> File
> "D:\Anaconda\envs\assess\lib\site-packages\sqlalchemy\orm\attributes.py",
> line 239, in __get__
> return self.impl.get(instance_state(instance), dict_)
> File
> "D:\Anaconda\envs\assess\lib\site-packages\sqlalchemy\orm\attributes.py",
> line 591, in get
> value = self.callable_(state, passive)
> File
> "D:\Anaconda\envs\assess\lib\site-packages\sqlalchemy\orm\strategies.py",
> line 507, in _load_for_state
> (orm_util.state_str(state), self.key)
> sqlalchemy.orm.exc.DetachedInstanceError: Parent instance <ConfigParam at
> 0x307d898> is not bound to a Session; lazy load operation of attribute
> 'config' cannot proceed
>
> I'm somewhat new to ORM's -
> naively I had assumed the backref should/could be loaded at the same time
> the parent is eagerly loaded, since all required information is retrieved
> in the above query.
> Is there anything I'm missing here?
>
> ps: This is on sqlalchemy 0.9.8 - if that should matter.
>
>
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