[sqlalchemy] How to order_by relation by another join?
Let's say I've got simple structure Order--Item--Detail.
class Detail(Base):
...
class Order(Base):
...
class Item(Base):
...
detail = relation(Detail, uselist=False, lazy=False)
order = relation(Order, uselist=False, backref='items')
Of course I can specify order_by for Order.items by any columns from
Item, but is there a way I can order_by a column defined in Detail?
I've naively tried something as backref('items', order_by=Detail.id),
but because Detail is anonymously joined to Item as e.g. 'details_1',
I've received ProgrammingError: invalid reference to FROM-clause
entry for table details.
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Re: [sqlalchemy] How to order_by relation by another join?
On Feb 10, 2010, at 10:17 AM, Andrija Zarić wrote:
Let's say I've got simple structure Order--Item--Detail.
class Detail(Base):
...
class Order(Base):
...
class Item(Base):
...
detail = relation(Detail, uselist=False, lazy=False)
order = relation(Order, uselist=False, backref='items')
Of course I can specify order_by for Order.items by any columns from
Item, but is there a way I can order_by a column defined in Detail?
I've naively tried something as backref('items', order_by=Detail.id),
but because Detail is anonymously joined to Item as e.g. 'details_1',
I've received ProgrammingError: invalid reference to FROM-clause
entry for table details.
The Order.items collection doesn't have the ability to order by a remote column
in the collection, unless you set the order_by to a subquery that joined out to
the ultimate target you care about.
Maybe, I haven't tried this, you could make an alternate (non primary) mapping
to Item that was a join of Item and Detail, i.e. like:
itemdetail = mapper(Item.__table__.join(Detail.__table__), non_primary=True)
Order.items = relation(itemdetail, order_by=itemdetail.c.detail_id)
I'm not 100% sure the non-primary mapper allowing additional attributes, and it
might need to be a mapping of a select().select_from(join) and not the join
directly, but if you have some time to experiment you might get something out
of that.
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Re: [sqlalchemy] How to order_by relation by another join?
On Feb 10, 2010, at 10:34 AM, Michael Bayer wrote:
Maybe, I haven't tried this, you could make an alternate (non primary)
mapping to Item that was a join of Item and Detail, i.e. like:
itemdetail = mapper(Item.__table__.join(Detail.__table__), non_primary=True)
Order.items = relation(itemdetail, order_by=itemdetail.c.detail_id)
I'm not 100% sure the non-primary mapper allowing additional attributes, and
it might need to be a mapping of a select().select_from(join) and not the
join directly, but if you have some time to experiment you might get
something out of that.
Here, I think it works:
from sqlalchemy import *
from sqlalchemy.orm import *
from sqlalchemy.ext.declarative import declarative_base
engine = create_engine('sqlite://', echo=True)
Base = declarative_base()
class Detail(Base):
__tablename__ = 'detail'
id = Column(Integer, primary_key=True)
item_id = Column(Integer, ForeignKey('item.id'))
class Order(Base):
__tablename__ = 'order'
id = Column(Integer, primary_key=True)
class Item(Base):
__tablename__ = 'item'
id = Column(Integer, primary_key=True)
order_id = Column(Integer, ForeignKey('order.id'))
detail = relation(Detail, uselist=False, lazy=False)
order = relation(Order, uselist=False)
j = Item.__table__.join(Detail.__table__)
itemdetail = mapper(Item, j, non_primary=True)
Order.items = relation(itemdetail, order_by=j.c.detail_id, viewonly=True)
metadata = Base.metadata
metadata.create_all(engine)
Session = scoped_session(sessionmaker(bind=engine))
Session.query(Order).options(eagerload(Order.items)).all()
the query is:
SELECT order.id AS order_id, anon_1.item_id AS anon_1_item_id,
anon_1.detail_id AS anon_1_detail_id, anon_1.item_order_id AS
anon_1_item_order_id, anon_1.detail_item_id AS anon_1_detail_item_id
FROM order LEFT OUTER JOIN (SELECT item.id AS item_id, item.order_id AS
item_order_id, detail.id AS detail_id, detail.item_id AS detail_item_id
FROM item JOIN detail ON item.id = detail.item_id) AS anon_1 ON order.id =
anon_1.item_order_id ORDER BY anon_1.detail_id
However I can't currently get back_populates or backref to relate the two
sides together. which isn't necessary if you dont need it.
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