Eric Ongerth wrote:
> If anyone out there has already implemented a custom DictOfLists
> collection class to map scalar keys to lists of values, I would be
> grateful for the opportunity to avoid reinventing the wheel. If not,
> I guess I'll start working on it.
>
> I've experimented successfully with attribute_mapped_collection and
> column_mapped_collection and they work just great. However, instead
> of mapping keys to single values, I need some of my mapper relations
> to map to a list of values.
>
> Here is more of the picture, for example.
>
> foos = Table('foos', metadata,
> Column('id', Integer, primary_key=True),
> Column('name', String(20)))
>
> bars = Table('bars', metadata,
> Column('id', Integer, primary_key=True),
> Column('foo_id', Integer, ForeignKey('foos.id')),
> Column('value', String(20)),
> Column('parent_id', Integer, ForeignKey('bars.id')))
>
> class Foo(object): pass
> class Bar(object): pass
>
> mapper(Foo, foos)
>
> mapper(Bar, bars, properties={
> 'foo':relation(Foo, uselist=False, backref='bars'),
> 'children':relation(Bar,
> backref=backref('parent',
> remote_side=[bars.c.id]))
> })
>
>
> ... So we have a relation of 1 Foo : many Bars. And within the Bars
> we also have 'adjacency' (tree-like) relations between the various
> rows of the 'bars' table. A Bar's children are kept in the standard
> list-like collection class.
>
> But what I really need is a *dict* instead of a list. Ok, SA already
> takes care of that. But I actually need a list-like collection to
> appear as the value for each key in the dict. Specifically, I need
> each Bar to be able to have stored children *per Foo*. And not keyed
> by the parent's foo, but the child's foo.
>
> Does that make sense? I'll be working on this immediately, but if
> anyone can shorten my path to getting this straight I'd be very glad.
> I'm beginning to work out the use of a custom collection_class for
> this, but I haven't done all that much with metaclassing and the way
> forward isn't obvious (the SA instructions about this seem to assume
> the programmer is pretty experienced with custom subclassing etc.)
You've spurred me on to finish a "collections of collections" article
I've been working on. Part one is up and hopefully will shed some light
on how these sorts of collections can be composed.
http://blog.discorporate.us/2008/02/sqlalchemy-partitioned-collections-1/
The whole collections toolkit is new in 0.4 and as you've discovered,
doesn't have a large body of documentation yet.
Cheers,
Jason
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