On Nov 13, 2012, at 11:03 PM, Bobby Impollonia wrote:
> I have constructed a sample program consisting of two mapped classes (using
> sqlalchemy.ext.declarative) that have a relationship/ backref between them.
> At runtime the program does the following:
> 1) Print whether the parent class has an attribute for its relationship to
> the child (declared as the backref)
> 2) Construct a child object
> 3) Repeat step 1
>
> The result (with SQLA 0.7.9) is that it prints 'False' during step 1 and then
> 'True' during step 3. I would expect True to be printed both times.
>
> Here is the full source of the program:
> https://gist.github.com/4070161
>
> Why does the property not exist when the first print statement executes?
"addresses" is generated on the Person class when the mappers enter the
"configuration" step, which is an automatically invoked process which occurs
when a mapping is first used. this process is deferred until a point at which
it's safe to assume all mappings are present, so that relationship()
directives, which refer to other mappings, can proceed to reconcile the
mappings they point to - otherwise by definition one of the mappings/classes
(if using declarative) doesn't exist yet for relationship/backref.
the process can be manually invoked via configure_mappers():
if __name__ == '__main__':
from sqlalchemy.orm import configure_mappers
configure_mappers()
print hasattr(Person, 'addresses')
Address()
print hasattr(Person, 'addresses')
>
> Thanks for any guidance.
>
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