Backref confusion

[Bobby Impollonia]

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?

Thanks for any guidance.

[Michael Bayer]

"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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[Bobby Impollonia]

Thank you! That is exactly the function I needed.