Discuss usage/design issues for SQLAlchemy, a Python SQL toolkit

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Sergey Kucheryavski | 31 Jul 2012 17:42

aliased in mixins

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))

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!

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Michael Bayer | 1 Aug 2012 03:46

Re: aliased in mixins

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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Sergey Kucheryavski | 1 Aug 2012 13:20

Re: aliased in mixins

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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Michael Bayer | 1 Aug 2012 16:20

Re: aliased in mixins

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

<at> 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))

On Aug 1, 2012, at 7:20 AM, Sergey Kucheryavski wrote:

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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Sergey Kucheryavski | 1 Aug 2012 19:52

Re: aliased in mixins

Thank you very much!

On Wednesday, August 1, 2012 4:20:23 PM UTC+2, Michael Bayer wrote:

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

<at> 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))

On Aug 1, 2012, at 7:20 AM, Sergey Kucheryavski wrote:

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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