On 8/24/15 7:41 PM, Douglas Russell wrote:
|
def print_stuff(mapper, class_):
print getattr(class_, 'books')
event.listen(mapper, 'mapper_configured', print_stuff)
author = Author(name='Chuck Paluhniuk')
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When I run this code, the result is that when Author is first used,
the 'mapper_configured' event will fire and my print_stuff function
callback will run.
The results is this:
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Traceback (most recent call last):
File "marshmallow_experiment.py", line 37, in <module>
author = Author(name='Chuck Paluhniuk')
File "<string>", line 2, in __init__
File
"/usr/local/Cellar/python/2.7.10/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/sqlalchemy/orm/instrumentation.py",
line 347, in _new_state_if_none
state = self._state_constructor(instance, self)
File
"/usr/local/Cellar/python/2.7.10/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/sqlalchemy/util/langhelpers.py",
line 747, in __get__
obj.__dict__[self.__name__] = result = self.fget(obj)
File
"/usr/local/Cellar/python/2.7.10/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/sqlalchemy/orm/instrumentation.py",
line 177, in _state_constructor
self.dispatch.first_init(self, self.class_)
File
"/usr/local/Cellar/python/2.7.10/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/sqlalchemy/event/attr.py",
line 258, in __call__
fn(*args, **kw)
File
"/usr/local/Cellar/python/2.7.10/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/sqlalchemy/orm/mapper.py",
line 2792, in _event_on_first_init
configure_mappers()
File
"/usr/local/Cellar/python/2.7.10/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/sqlalchemy/orm/mapper.py",
line 2691, in configure_mappers
mapper, mapper.class_)
File
"/usr/local/Cellar/python/2.7.10/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/sqlalchemy/event/attr.py",
line 218, in __call__
fn(*args, **kw)
File
"/usr/local/Cellar/python/2.7.10/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/sqlalchemy/orm/events.py",
line 527, in wrap
fn(*arg, **kw)
File "marshmallow_experiment.py", line 33, in print_stuff
print getattr(class_, 'books')
AttributeError: type object 'Author' has no attribute 'books'
|
Should not the backref be configured at that point? What I'm trying to
do is use the mapper_configured event in order to autocreate
marshamallow-sqlalchemy schemas, which works fine except that because
books doesn't exist at the point when the callback function runs, the
resulting schema is also missing the books part.
If I do this instead:
|
author = Author(name='Chuck Paluhniuk')
print_stuff(None, Author)
|
Then the result is:
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Author.books
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so you can see what's happening here. Author.books is being set up,
but not within the event. In this case Author is being configured
before Book, and as far as it knows, that's it, it's done. The event is
called. Book hasn't been touched so therefore neither has "books".
Only outside of the whole thing, where Book also got set up, does your
inspection work.
The solution is to either use back_populates instead of backref, which
is probably the only way the ORM would have done it today if starting
again, or use the "after_configured()" event
http://docs.sqlalchemy.org/en/rel_1_0/orm/events.html#sqlalchemy.orm.events.MapperEvents.after_configured
which is intentionally here so that you can do things that require all
known mappers to be fully set up, which is the case here.
As is so often the case I'm horrified the API docs don't have cross
linking here. doing that..
I.e. It is now there.
Thanks again for your continued excellent support.
Douglas
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