aliased in mixins

[Sergey Kucheryavski]

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))

And then use it like:

class Node(MPTT, Base):

    __tablename__ = 'node'

    id = db.Column(db.Integer, primary_key=True)

    name = db.Column(db.String(150), nullable = False)

    urlname = db.Column(db.String(150), nullable = False)

    is_visible = db.Column(db.Boolean, default = True)

    def __repr__(self):

        return '%r (%r - %r)' % (self.name, self.lft, self.rgt)

    def __init__(self, name, urlname, parent = None):

        super(Node, self).__init__(parent)

        self.name = name

        self.urlname = urlname

However I have problem using aliased in the mixin getting an error: 

  File "./app/ext_mptt/models.py", line 24, in ordernum

    mptt2 = aliased(cls)

  File "./venv/lib/python2.7/site-packages/sqlalchemy/orm/util.py", line 385, in aliased

    return AliasedClass(element, alias=alias, name=name, adapt_on_names=adapt_on_names)

  File "./venv/lib/python2.7/site-packages/sqlalchemy/orm/util.py", line 298, in __init__

    self.__mapper = _class_to_mapper(cls)

  File "./venv/lib/python2.7/site-packages/sqlalchemy/orm/util.py", line 673, in _class_to_mapper

    raise exc.UnmappedClassError(class_or_mapper)

sqlalchemy.orm.exc.UnmappedClassError: Class 'app.cms.models.Node' is not mapped

I thought the num_children method should be called when Node is mapped. Will be grateful for any hints!

[Michael Bayer]

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

[Sergey Kucheryavski]

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. 

[Michael Bayer]

oh, sorry, that's a mixin.  this combination of variables is not supported at this time.  You need to use an event:

from sqlalchemy import event

from sqlalchemy.orm import mapper

@event.listens_for(mapper, "mapper_configured")

def set_num_children(mapper, cls):

    if not issubclass(cls, MPTT):

        return

    mptt2 = aliased(cls)

    cls.num_children = column_property(

                select([func.count(cls.id)]).where(cls.parent_id == mptt2.id))

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[Sergey Kucheryavski]

Thank you very much!

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