[Sqlalchemy-tickets] Issue #4185: AssociationProxy is not properly initialized when created by a declared_attr of a mixin class (zzzeek/sqlalchemy)

[Eric A. (Omni) <iss...@bi...>]

New issue 4185: AssociationProxy is not properly initialized when created by a declared_attr of a mixin class
https://bitbucket.org/zzzeek/sqlalchemy/issues/4185/associationproxy-is-not-properly

Eric Atkin (Omni):

After upgrading from 1.1.14 to 1.2.2, my models broke. A simplified representation is included below that demonstrates the issue. If the Parent.children association proxy is not accessed before any subclasses are defined, it's owning_class attribute will be None. This caused the following traceback, but I'm sure it would be the root cause of many more issues throughout the library:

```
Traceback (most recent call last):
  File "{...}/.venv/lib/python3.6/site-packages/pyramid_mako/__init__.py", line 148, in __call__
    result = template.render_unicode(**system)
  File "{...}/.venv/lib/python3.6/site-packages/mako/template.py", line 471, in render_unicode
    as_unicode=True)
  File "{...}/.venv/lib/python3.6/site-packages/mako/runtime.py", line 838, in _render
    **_kwargs_for_callable(callable_, data))
  File "{...}/.venv/lib/python3.6/site-packages/mako/runtime.py", line 873, in _render_context
    _exec_template(inherit, lclcontext, args=args, kwargs=kwargs)
  File "{...}/.venv/lib/python3.6/site-packages/mako/runtime.py", line 899, in _exec_template
    callable_(context, *args, **kwargs)
  File "{...}/dispatch/templates/base/base.mako", line 297, in render_body
    <%block name="content">
  File "{...}/dispatch/templates/base/detail_carriage.mako", line 710, in render_content
    % if editable or _context.child_invoice_meta:
  File "{...}/sqlalchemy_auth/block_base.py", line 86, in __getattribute__
    return super().__getattribute__(name)
  File "{...}/.venv/lib/python3.6/site-packages/sqlalchemy/ext/associationproxy.py", line 263, in __get__
    if self.scalar:
  File "{...}/.venv/lib/python3.6/site-packages/sqlalchemy/util/langhelpers.py", line 767, in __get__
    obj.__dict__[self.__name__] = result = self.fget(obj)
  File "{...}/.venv/lib/python3.6/site-packages/sqlalchemy/ext/associationproxy.py", line 233, in scalar
    scalar = not self._get_property().uselist
  File "{...}/.venv/lib/python3.6/site-packages/sqlalchemy/ext/associationproxy.py", line 215, in _get_property
    return (orm.class_mapper(self.owning_class).
  File "{...}/.venv/lib/python3.6/site-packages/sqlalchemy/orm/base.py", line 426, in class_mapper
    raise exc.UnmappedClassError(class_)
UnmappedClassError: Class 'builtins.NoneType' is not mapped
```

```
#!python

from sqlalchemy import Column, ForeignKey, Integer, Text
from sqlalchemy.ext.associationproxy import association_proxy
from sqlalchemy.ext.declarative import declared_attr, declarative_base
from sqlalchemy.orm import backref, relationship
from sqlalchemy.orm.collections import attribute_mapped_collection

Declarative_Base = declarative_base()


class Mixin:
    @declared_attr
    def children(cls):
        class Child(Declarative_Base):
            parent_id = Column(Integer, ForeignKey(f'{cls.__tablename__}.id'), primary_key=True)
            key = Column(Text, primary_key=True)
            value = Column(Text, nullable=False)
            
            parent = relationship(cls, backref=backref(
                '_children',
                cascade='all, delete-orphan',
                collection_class=attribute_mapped_collection('key')
            ))
            
            __tablename__ = f'{cls.__tablename__}_child'
        
        return association_proxy(
            '_children',
            'value',
            creator=lambda k, v: Child(key=k, value=v),
        )


class Parent(Mixin, Declarative_Base):
    id = Column(Integer, primary_key=True)
    
    __tablename__ = 'parent'


#Parent.children  # uncomment this for workaround


class SubParent(Parent):
    id = Column(Integer, ForeignKey(Parent.id), primary_key=True)
    
    __tablename__ = 'subparent'


Parent.children.owning_class  # <class 'NoneType'>

```

This seems to be caused by the fix for https://bitbucket.org/zzzeek/sqlalchemy/issues/4116/accessing-associationproxy-of-aliased
in commit https://bitbucket.org/zzzeek/sqlalchemy/commits/f14a58dea4b825beb4baaef44389880927543cc4

I don't know how to fix this, but am able to work around it for now by accessing the proxy attribute prior to the subclass definitions. I hope that by demonstrating it, you would be able to design a change that addresses the original issue and mitigates this side-effect.

Thank you,
Eric

Responsible: zzzeek