On Aug 9, 2011, at 8:38 AM, Wichert Akkerman wrote:
> I have a model where I have articles and images, with a many-to-many relation
> between them. Since the order of images for an article is important this
> relation should be ordered from the article site. My naieve implementation
> looks like this:
>
> article_images = Table('article_image', BaseObject.metadata,
> Column('article_id', Integer(),
> ForeignKey('article.id',
> onupdate='CASCADE', ondelete='CASCADE'),
> primary_key=True),
> Column('image_id', Integer(),
> ForeignKey('image.id',
> onupdate='CASCADE', ondelete='CASCADE'),
> primary_key=True),
> Column('position', Integer()))
>
>
> class Image(BaseObject):
> __tablename__ = 'image'
> id = Column(Integer(),
> Sequence('image_id_seq', optional=True),
> primary_key=True, autoincrement=True)
> path = Column(String(128), nullable=False, unique=True)
>
>
> class Article(BaseObject):
> __tablename__ = 'article'
> id = Column(Integer(),
> Sequence('article_id_seq', optional=True),
> primary_key=True, autoincrement=True)
>
> #: An ordered list of images for this article. The first image
> #: is considered to be the *key* image.
> images = relationship(Image,
> order_by=[article_images.c.position],
> secondary=article_images,
> collection_class=ordering_list('position'))
>
>
> Unfortunately this breaks since OrderingList assumes that the position
> attribute is set on Image instead of the article_images table. From what I
> can see this is not easily fixed since OrderingList only gets the list of
> Image instances and doesn't have access to the related article_images row. Is
> there another way to accomplish this?
Well since the m2m table has a meaningful column on it "position", that means
you'd go into the association object pattern, optionally though likely with an
association proxy for Article.images = association_proxy('articleimages',
'image'), and the ordering_list goes on the "articleimages" relationship from
Article->ArticleImage.
I've spent all weekend updating docs so lookup "association object" in the
relationship config chapter. I also rewrote most of "association proxy".
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