Thanks Michael, I greatly appreciate your help. But I am still a bit confused. To set the attribute I need to have the alias for MPTT (or, actually any class that inherits MPTT mixin class) table. But I can not use aliased() neither inside the MPTT definition nor after it — in both cases I get the mapping error.
On Wednesday, August 1, 2012 3:46:56 AM UTC+2, Michael Bayer wrote: > > > On Jul 31, 2012, at 11:42 AM, Sergey Kucheryavski wrote: > > I would like to use Mixin to make a base class for MPTT trees, just as an > example > > class MPTT(object): > @declared_attr > def __tablename__(cls): > return cls.__name__.lower() > > id = db.Column(db.Integer, primary_key = True) > level = db.Column(db.Integer, nullable = False) > lft = db.Column(db.Integer, nullable = False) > rgt = db.Column(db.Integer, nullable = False) > > @declared_attr > def parent_id(cls): > return db.Column(db.Integer, db.ForeignKey(cls.id)) > > @declared_attr > def num_children(cls): > mptt2 = aliased(cls) > return > column_property(select([func.count(cls.id)]).where(cls.parent_id > == mptt2.id)) > > > > yeah num_children() is evaluated within declarative's metaclass, before > MPTT is mapped. So you have to set that attribute on after the fact: > > mptt2 = aliased(cls) > MPTT.num_children = column_property(....) > > docs (see the last example): > > > http://docs.sqlalchemy.org/en/rel_0_7/orm/mapper_config.html#using-column-property > > > -- You received this message because you are subscribed to the Google Groups "sqlalchemy" group. To view this discussion on the web visit https://groups.google.com/d/msg/sqlalchemy/-/3tDxuZQ9VbwJ. To post to this group, send email to sqlalchemy@googlegroups.com. To unsubscribe from this group, send email to sqlalchemy+unsubscr...@googlegroups.com. For more options, visit this group at http://groups.google.com/group/sqlalchemy?hl=en.
