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):

    <at> 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)

    <at> declared_attr

    def parent_id(cls):

        return db.Column(db.Integer, db.ForeignKey(cls.id))

    <at> 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

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