SQLAlchemy - relationship limited on more than just the foreign key

Posted by Marian on Stack Overflow See other posts from Stack Overflow or by Marian
Published on 2010-05-29T15:22:03Z Indexed on 2010/05/31 8:42 UTC
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Filed under:

python

|

sqlalchemy

|

relationship

|

database-structure

I have a wiki db layout with Page and Revisions. Each Revision has a page_id referencing the Page, a page relationship to the referenced page; each Page has a all_revisions relationship to all its revisions. So far so common.

But I want to implement different epochs for the pages: If a page was deleted and is recreated, the new revisions have a new epoch. To help find the correct revisions, each page has a current_epoch field. Now I want to provide a revisions relation on the page that only contains its revisions, but only those where the epochs match.

This is what I've tried:

revisions = relationship('Revision',
    primaryjoin = and_(
        'Page.id == Revision.page_id',
        'Page.current_epoch == Revision.epoch',
    ),
    foreign_keys=['Page.id', 'Page.current_epoch']
)

Full code (you may run that as it is)

However this always raises ArgumentError: Could not determine relationship direction for primaryjoin condition ...`, I've tried all I had come to mind, it didn't work.

What am I doing wrong? Is this a bad approach for doing this, how could it be done other than with a relationship?

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