How to properly structure a self-referential SQLalchemy Adjacency List Mixin

Question

I am trying to write a mixin for SQLalchemy which will define a parent/children self referential adjacency relationship on the target class. I am using flask.ext.appbuilder for the project scaffolding. My mixin looks like:

import inflection
from sqlalchemy import Column, ForeignKey, Integer
from sqlalchemy.ext.declarative import declared_attr
from sqlalchemy.orm import backref, relationship

class SimpleTreeBaseMixin(NameBaseMixin):

"""A mixin which organizes target class in a simple tree structure.

Bases:
    .basemixins.NameBaseMixin

Attributes:
    SEPARATOR (str): Separator used to divide full name.

    parent_id (Optional[int]): Primary key of parent object (if any).
    parent (obj): The parent object.
    children (list[obj]): Collection of related children objects

"""

SEPARATOR = '.'

@declared_attr
def parent_id(cls):
    fk = '{}.id'.format(inflection.tableize(cls.__name__))
    return Column(Integer, ForeignKey(fk))

@declared_attr
def children(cls):
    return relationship(cls.__name__,
                        backref=backref('parent', remote_side=cls.id),
                        cascade='all, delete-orphan')

def __init__(self):
    super(SimpleTreeMixin, self).__init__()

def __repr__(self):
    return self.full_name

@property
def full_name(self):
    if self.parent:
        return self.parent.full_name + self.SEPARATOR + self.name
    else:
        return self.name

However, when I apply this mixin to a class like, the following (note that attributes id , name and notes as well as the __tablename__ come from NameBaseMixin ):

from flask.ext.appbuilder import Model

class PartyCategory(SimpleTreeBaseMixin, Model):

"""Represent a party category.

Categories can be put into a tree structure by defining parent and
children PartyCategory objects.

Bases:
    .basemixins.SimpleTreeBaseMixin
    flask.ext.appbuilder.Model

Attributes:
    id (int): Primary key of category.
    name (str): Name of category.

    parent_id (Optional[int]): Primary key of parent object (if any).
    parent (obj): The parent object.
    children (list[obj]): Collection of related children objects.

    notes (Optional[str]): internal notes about given object.

"""

def __init__(self):
    super(PartyCategory, self).__init__()

and then try to build the database, I get an sqlalchemy.exc.ArgumentError :

Traceback (most recent call last):
  File "run.py", line 4, in <module>
    from app import app
  File "C:\Code\Projects\Fluffheads\app\__init__.py", line 42, in <module>
    from . import views
  File "C:\Code\Projects\Fluffheads\app\views.py", line 53, in <module>
    class PartyView(ExtendedModelView):
  File "C:\Code\Projects\Fluffheads\app\views.py", line 57, in PartyView
    datamodel = SQLAInterface(Party)
  File "C:\Python27\lib\site-packages\flask_appbuilder\models\sqla\interface.py", line 46, in __init__
    for prop in sa.orm.class_mapper(obj).iterate_properties:
  File "C:\Python27\lib\site-packages\sqlalchemy\orm\base.py", line 421, in class_mapper
    mapper = _inspect_mapped_class(class_, configure=configure)
  File "C:\Python27\lib\site-packages\sqlalchemy\orm\base.py", line 400, in _inspect_mapped_class
    mapper._configure_all()
  File "C:\Python27\lib\site-packages\sqlalchemy\orm\mapper.py", line 1167, in _configure_all
    configure_mappers()
  File "C:\Python27\lib\site-packages\sqlalchemy\orm\mapper.py", line 2756, in configure_mappers
    mapper._post_configure_properties()
  File "C:\Python27\lib\site-packages\sqlalchemy\orm\mapper.py", line 1710, in _post_configure_properties
    prop.init()
  File "C:\Python27\lib\site-packages\sqlalchemy\orm\interfaces.py", line 183, in init
    self.do_init()
  File "C:\Python27\lib\site-packages\sqlalchemy\orm\relationships.py", line 1616, in do_init
    self._generate_backref()
  File "C:\Python27\lib\site-packages\sqlalchemy\orm\relationships.py", line 1847, in _generate_backref
    mapper._configure_property(backref_key, relationship)
  File "C:\Python27\lib\site-packages\sqlalchemy\orm\mapper.py", line 1613, in _configure_property
    prop.init()
  File "C:\Python27\lib\site-packages\sqlalchemy\orm\interfaces.py", line 183, in init
    self.do_init()
  File "C:\Python27\lib\site-packages\sqlalchemy\orm\relationships.py", line 1613, in do_init
    self._setup_join_conditions()
  File "C:\Python27\lib\site-packages\sqlalchemy\orm\relationships.py", line 1688, in _setup_join_conditions
    can_be_synced_fn=self._columns_are_mapped
  File "C:\Python27\lib\site-packages\sqlalchemy\orm\relationships.py", line 1965, in __init__
    self._check_remote_side()
  File "C:\Python27\lib\site-packages\sqlalchemy\orm\relationships.py", line 2433, in _check_remote_side
    'the relationship.' % (self.prop, ))
sqlalchemy.exc.ArgumentError: Relationship PartyCategory.parent could not determine any unambiguous local/remote column pairs based on join condition
and remote_side arguments.  Consider using the remote() annotation to accurately mark those elements of the join condition that are on the remote side
 of the relationship.

I also tried passing the remote_side as a string, ie remote_side='cls.id' but that throws a sqlalchemy.exc.InvalidRequestError :

Traceback (most recent call last):
  File "run.py", line 4, in <module>
    from app import app
  File "C:\Code\Projects\Fluffheads\app\__init__.py", line 42, in <module>
    from . import views
  File "C:\Code\Projects\Fluffheads\app\views.py", line 53, in <module>
    class PartyView(ExtendedModelView):
  File "C:\Code\Projects\Fluffheads\app\views.py", line 57, in PartyView
    datamodel = SQLAInterface(Party)
  File "C:\Python27\lib\site-packages\flask_appbuilder\models\sqla\interface.py", line 46, in __init__
    for prop in sa.orm.class_mapper(obj).iterate_properties:
  File "C:\Python27\lib\site-packages\sqlalchemy\orm\base.py", line 421, in class_mapper
    mapper = _inspect_mapped_class(class_, configure=configure)
  File "C:\Python27\lib\site-packages\sqlalchemy\orm\base.py", line 400, in _inspect_mapped_class
    mapper._configure_all()
  File "C:\Python27\lib\site-packages\sqlalchemy\orm\mapper.py", line 1167, in _configure_all
    configure_mappers()
  File "C:\Python27\lib\site-packages\sqlalchemy\orm\mapper.py", line 2756, in configure_mappers
    mapper._post_configure_properties()
  File "C:\Python27\lib\site-packages\sqlalchemy\orm\mapper.py", line 1710, in _post_configure_properties
    prop.init()
  File "C:\Python27\lib\site-packages\sqlalchemy\orm\interfaces.py", line 183, in init
    self.do_init()
  File "C:\Python27\lib\site-packages\sqlalchemy\orm\relationships.py", line 1616, in do_init
    self._generate_backref()
  File "C:\Python27\lib\site-packages\sqlalchemy\orm\relationships.py", line 1847, in _generate_backref
    mapper._configure_property(backref_key, relationship)
  File "C:\Python27\lib\site-packages\sqlalchemy\orm\mapper.py", line 1613, in _configure_property
    prop.init()
  File "C:\Python27\lib\site-packages\sqlalchemy\orm\interfaces.py", line 183, in init
    self.do_init()
  File "C:\Python27\lib\site-packages\sqlalchemy\orm\relationships.py", line 1612, in do_init
    self._process_dependent_arguments()
  File "C:\Python27\lib\site-packages\sqlalchemy\orm\relationships.py", line 1637, in _process_dependent_arguments
    setattr(self, attr, attr_value())
  File "C:\Python27\lib\site-packages\sqlalchemy\ext\declarative\clsregistry.py", line 293, in __call__
    (self.prop.parent, self.arg, n.args[0], self.cls)
sqlalchemy.exc.InvalidRequestError: When initializing mapper Mapper|PartyCategory|party_categories, expression 'cls.id' failed to locate a name ("name
 'cls' is not defined"). If this is a class name, consider adding this relationship() to the <class 'app.models.party.PartyCategory'> class after both
 dependent classes have been defined.

How can I properly structure the children relationship so that it can be applied to target classes?

Answer 1

So, the simple solution was to pass the remote_side of the parent relationship as a string with the class name like this:

@declared_attr
def children(cls):
    return relationship(
        cls.__name__,
        backref=backref(
            'parent',
            remote_side='{}.id'.format(cls.__name__)
        ),
        cascade='all, delete-orphan'
    )