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.
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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