I know ive put many folks through the wringer dealing with this
function, in order to get their inherited mappers to load instances
polymorphically. It seems like we probably dont need it for the most
common case, which is that you are using only joined-table
inheritance from a single common base table (i.e. no concrete-
inheritance), and have a specific column in your base table that
indicates the "polymorphic type". if thats the case, you can forego
the usage of polymorphic_union and just string together your tables
using joins and outerjoins. Theres a slight tweak in the post 0.3.8
trunk with eager loading to get "eager loading to a polymorphic
mapper" to work with this case.
meaning, you can make mappers like this:
page_join = page_table.outerjoin(magazine_page_table).outerjoin
(classified_page_table)
magazine_join = page_table.join(magazine_page_table).outerjoin
(classified_page_table)
page_mapper = mapper(Page, page_table, select_table=page_join,
polymorphic_on=page_table.c.type, polymorphic_identity='p')
magazine_page_mapper = mapper(MagazinePage, magazine_page_table,
select_table=magazine_join, inherits=page_mapper,
polymorphic_identity='m')
classified_page_mapper = mapper(ClassifiedPage,
classified_page_table, inherits=magazine_page_mapper,
polymorphic_identity='c')
full example attached.
conclusions we can draw from this ? I am really, really dumb for not
seeing this for like, the past year. the queries are a lot easier to
create and read, plus we avoid all the other issues UNIONS bring
forth (like same column name but different types colliding, etc).
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from sqlalchemy import *
metadata = BoundMetaData('sqlite://', echo=True)
page_table = Table('page', metadata,
Column('id', Integer, primary_key=True, default=None),
Column('page_no', Integer),
Column('type', CHAR(1)),
)
magazine_page_table = Table('magazine_page', metadata,
Column('page_id', Integer, ForeignKey('page.id'), primary_key=True),
Column('orders', String(45)),
)
classified_page_table = Table('classified_page', metadata,
Column('magazine_page_id', Integer, ForeignKey('magazine_page.page_id'), primary_key=True),
Column('titles', String(45)),
)
metadata.create_all()
page_join = page_table.outerjoin(magazine_page_table).outerjoin(classified_page_table)
magazine_join = page_table.join(magazine_page_table).outerjoin(classified_page_table)
class Page(object):
def __init__(self, **kwargs):
for key, value in kwargs.iteritems():
setattr(self, key, value)
def __repr__(self):
return "%s(%s)" % (self.__class__.__name__, ','.join(["%s=%s" % (k, repr(v)) for k, v in self.__dict__.iteritems() if k[0] != '_']))
class MagazinePage(Page):pass
class ClassifiedPage(MagazinePage):pass
page_mapper = mapper(Page, page_table, select_table=page_join, polymorphic_on=page_table.c.type, polymorphic_identity='p')
magazine_page_mapper = mapper(MagazinePage, magazine_page_table, select_table=magazine_join, inherits=page_mapper, polymorphic_identity='m')
classified_page_mapper = mapper(ClassifiedPage, classified_page_table, inherits=magazine_page_mapper, polymorphic_identity='c')
sess = create_session()
objs = [Page(page_no=5), MagazinePage(page_no=6, orders='some text'), ClassifiedPage(page_no=7, orders='some other text', titles='classified titles')]
for o in objs:
sess.save(o)
sess.flush()
sess.clear()
print sess.query(Page).list()
print sess.query(MagazinePage).list()
