On May 24, 2010, at 10:38 PM, chrysn wrote:
> hello sqlalchemy,
>
> i'm setting up a data structure that builds a group of trees using
> combined primary keys. i got it to work (with help from rmancy on irc),
> but it is far from intuitive.
>
> my code looks like this:
>
> ql = Table('ql', metadata,
> # that's not the original column sort order, but it shouldn't
> matter
> Column(u'qid', Integer, primary_key=True),
> Column(u'lang', String(5), primary_key=True),
> Column(u'version', Integer, primary_key=True),
> Column(u'reflang', String(5), ForeignKey('ql.lang')),
> Column(u'refversion', Integer, ForeignKey('ql.version')),
> Column(...),
> )
>
>
> class QL(object):
> def __repr__(self):
> return '<QL (%s, %s, %s)>'%(self.qid, self.lang, self.version)
>
>
> mapper(QL, ql, properties={
> ...,
> 'ref': relationship(
> QL,
> primaryjoin=and_(
> ql.c.qid==ql.c.qid,
> ql.c.reflang==ql.c.lang,
> ql.c.refversion==ql.c.version),
> backref=backref('backref'),
> foreign_keys=[ql.c.qid, ql.c.reflang, ql.c.refversion],
> remote_side=[ql.c.qid, ql.c.lang, ql.c.version]),
> })
>
> my problem is that the primaryjoin query is rather unintellegible --
> which side is which? i tried to substitude the right sides of the
> primaryjoin and the remote_side items with referenced_ql =
> ql.alias('referenced_ql'), but that made the query construction fail
> with "Could not locate any equated, locally mapped column pairs for
> primaryjoin condition ...".
>
> is there any way to express this more clearly?
this is a very rare use case, in that your primary join requires the equation
of a column to itself. So the hint provided by remote_side is required. The
foreign keys and primaryjoin hints should not be needed here, however.
There are examples of various varieties of configuration for this type of join
in http://www.sqlalchemy.org/trac/browser/test/orm/test_relationships.py#L104 .
You should be able to use the style in `test_very_implicit`, i.e. only
remote_side on the many-to-one side of the relationship().
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