well at the moment this is how it has to work, since youre trying to
achieve the polymorphic effect at the level of every mapper....this
is not *so* typical of a use case, its a little more ambitious. i
dont think its going to reveal too many more issues than a simplistic
inheritance relationship would raise.
theres a ticket to get the polymorphic query to be automatically
generated, instead of using the "polymorphic union" explicitly, which
would probably help. this enhancement relies upon the fact that
mappers are compiled after all of them are constructed...a feature
which has taken me three minor versions so far to get (close to)
right :).
hibernate's approach to polymorphism doesnt use UNIONs, it just
issues distinct queries for everything. I suppose if i had gone with
that, some of these decisions might be easier (or maybe not).
apparently JSR-220 (based on hibernate) actually calls for all
mappers to be completely polymorphic, such that if you issue an open
query on "java.lang.Object", it will load every object of every type
in the entire database ! that seems kind of wacky to me, more of an
OODB type of thing.
On Jul 20, 2006, at 3:38 PM, Sol wrote:
> This time with attachement;)
>> Same old topic. This time i try to make a some what longer
>> polymorphic
>> hierarchy, like:
>>
>> Base <= Item <= Leaf
>>
>> The attachment is what i came up with. The first impression is
>> that it
>> works, but my confidence in it is limited. Further more it will
>> become
>> ugly if we add more hierarchies.
>>
>> Is there a more simplistic way to achieve this?
>>
>> Cheers, Sol.
> from sqlalchemy import *
>
> #db = create_engine('postgres:///satest')
> db = create_engine("sqlite:///:memory:")
>
> metadata = BoundMetaData(db)
>
>
> # classes
> class Base(object):
> def __repr__(self):
> return "%s %s" % (self.__class__.__name__, self.id)
>
> class Item(Base): pass
>
> class Leaf(Item): pass
>
> # tables
> base = Table('base', metadata,
> Column('id', Integer, primary_key = True),
> Column('type', String),
> )
>
> item = Table('item', metadata,
> Column('id', Integer, ForeignKey('base.id'), primary_key = True),
> )
>
> leaf = Table('leaf', metadata,
> Column('id', Integer, ForeignKey('item.id'), primary_key = True),
> )
>
> # mappers
> base_join = polymorphic_union(
> {
> 'leaf' : base.join(item).join(leaf),
> 'item' : base.join(item),
> 'base' : base.select(base.c.type=='base'),
> }, None, 'pjoin')
>
>
> base_mapper = mapper(Base, base, select_table = base_join,
> polymorphic_on = base_join.c.type,
> polymorphic_identity='base',
> )
>
>
> item_join = polymorphic_union(
> {
> 'leaf' : base.join(item).join(leaf),
> 'item' : base_join.select(base_join.c.type=='item'),
> }, None, 'pjoin')
>
>
> item_mapper = mapper(Item, item, select_table = item_join,
> polymorphic_on = base_join.c.type,
> inherits=base_mapper,
> polymorphic_identity='item')
>
> leaf_mapper = mapper(Leaf, leaf, inherits=item_mapper,
> polymorphic_identity='leaf')
>
>
> # create all tables
> metadata.create_all()
>
> # create a session
> #metadata.engine.echo = True
> session = create_session()
>
> session.save(Leaf())
> session.save(Leaf())
> session.save(Base())
> session.save(Item())
> session.save(Item())
> session.flush()
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