Hi again,
I'm trying to track an issue making use of Marshmallow-SQLAlchemy (I've
also seen what looks like a very similar problem in ColanderAlchemy, but I
haven't dug into that yet). I think it might have something to do with the
mapper_configured event and whether the class is truly fully configured at
that point. Possibly I'm misunderstanding what that means.
If I do something like this:
import sqlalchemy as sa
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import scoped_session, sessionmaker, relationship
from sqlalchemy import event
from sqlalchemy.orm import mapper
engine = sa.create_engine('sqlite:///:memory:')
session = scoped_session(sessionmaker(bind=engine))
Base = declarative_base()
class Author(Base):
__tablename__ = 'authors'
id = sa.Column(sa.Integer, primary_key=True)
name = sa.Column(sa.String)
def __repr__(self):
return '<Author(name={self.name!r})>'.format(self=self)
class Book(Base):
__tablename__ = 'books'
id = sa.Column(sa.Integer, primary_key=True)
title = sa.Column(sa.String)
author_id = sa.Column(sa.Integer, sa.ForeignKey('authors.id'))
author = relationship("Author", backref='books')
def __repr__(self):
return '<Book(title={self.title!r})>'.format(self=self)
Base.metadata.create_all(engine)
def print_stuff(mapper, class_):
print getattr(class_, 'books')
event.listen(mapper, 'mapper_configured', print_stuff)
author = Author(name='Chuck Paluhniuk')
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:
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:
Author.books
I.e. It is now there.
Thanks again for your continued excellent support.
Douglas
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