This is pretty amazing. I get it to *almost* work. The issue seems to be
that I don't use the database column names as my model attribute names. The
db column names are really obfuscated, so it's more like:
class Version(Base):
id = Column('vrsn_nbr', Integer, primary_key=True)
date = Column('dt', DateTime)
etc..
It seems when this relationship is populated, it creates a Version object,
but it sets all of the database column names as attributes, and the defined
Column names are all None. Is there a way to get around that?
On Fri, Jan 31, 2014 at 6:38 PM, Michael Bayer <mike...@zzzcomputing.com>wrote:
> well I can get it to work for lazy loading like this:
>
> expr = select([func.max(Version.id)]).\
> where(Version.object_id == Object.id).\
> correlate_except(Version).as_scalar()
>
> Object.current_version = relationship(Version,
> primaryjoin=and_(
> expr == Version.id,
> Version.object_id ==
> Object.id
> )
> )
>
>
> but for joined load, the criteria needs to fit into a LEFT OUTER JOIN ON
> clause. Both SQLite and Postgresql reject an aggregate function in the ON
> clause. So I didn’t really know how to get that because you have to
> think in terms of the SQL….but then the usual approach is that you need to
> JOIN to a subquery that has the aggregate inside of it. So I use instead
> the pattern you see here:
> http://docs.sqlalchemy.org/en/rel_0_9/orm/tutorial.html#using-subqueries
> which is the subquery of “X.foo_id, func.AGGREGATE(X.id)” that then joins
> to the parent table, and then I go with “non primary mapper”, a use case
> that I recently added to the documentation at
> http://docs.sqlalchemy.org/en/rel_0_9/orm/relationships.html#relationship-to-non-primary-mapper,
> even though this has been around for years.
>
> and it looks like……
>
> expr = select([
> func.max(Version.id).label("max_id"),
> Version.object_id
> ]).group_by(Version.object_id).alias()
>
> stmt = select([Version]).\
> select_from(join(Version, expr, Version.id == expr.c.max_id)).\
> alias()
>
> current_version = mapper(Version, stmt, non_primary=True)
>
> Object.current_version = relationship(current_version)
>
> I think I might have actually written a mapping like this as an example
> back in SQLAlchemy 0.1 even, this was the goofy kind of thing I thought
> everyone would be doing all the time.
>
> works with joinedload. Query is not too efficient, but is like:
>
> SELECT objects.id AS objects_id, anon_1.id AS anon_1_id, anon_1.object_id
> AS anon_1_object_id
> FROM objects LEFT OUTER JOIN (SELECT versions.id AS id,
> versions.object_id AS object_id
> FROM versions JOIN (SELECT max(versions.id) AS max_id, versions.object_id
> AS object_id
> FROM versions GROUP BY versions.object_id) AS anon_2 ON versions.id =
> anon_2.max_id) AS anon_1 ON objects.id = anon_1.object_id
> WHERE objects.id = %(id_1)s
>
>
>
>
> On Jan 31, 2014, at 5:35 PM, Josh Kuhn <deontologic...@gmail.com> wrote:
>
> I've got a two tables I'd like to create a relationship for. One is the
> object, and another tracks versions.
>
> Here's a gist with the setup:
> https://gist.github.com/deontologician/8744532
>
> Basically, the object doesn't have a direct reference to the current
> version stored in the table. Instead, the current version is defined as the
> maximum version that points to that object.
>
> I'd like to have a one-to-one "current_version" relationship, but this has
> proven difficult (at least in 0.8.4). The primary goal is to allow using
> the joinedload options to control populating the current_version field, but
> that only works when a relationship is defined and is non-dynamic.
>
> Any hints as to how to get this to work like I want?
>
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