[Sqlalchemy-tickets] Issue #4329: KeyError when using Association Objects with back_populates – example from documentation doesn't work (zzzeek/sqlalchemy)

[Lars B. <iss...@bi...>]

New issue 4329: KeyError when using Association Objects with back_populates – example from documentation doesn't work
https://bitbucket.org/zzzeek/sqlalchemy/issues/4329/keyerror-when-using-association-objects

Lars Blumberg:

[This is a cross post from StackOverflow](https://stackoverflow.com/questions/52101595/sqlalchemy-throwing-keyerror-when-using-association-objects-with-back-populates)

SQLAlchemy nicely documents how to use Association Objects with back_populates.

However, when copy-and-pasting the example from that documentation, adding children to a parent throws a KeyError as following code shows. The model classes are copied 100% from the documentation:

```
#!python

from sqlalchemy import Column, ForeignKey, Integer, String
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import relationship
from sqlalchemy.schema import MetaData

Base = declarative_base(metadata=MetaData())

class Association(Base):
    __tablename__ = 'association'
    left_id = Column(Integer, ForeignKey('left.id'), primary_key=True)
    right_id = Column(Integer, ForeignKey('right.id'), primary_key=True)
    extra_data = Column(String(50))
    child = relationship("Child", back_populates="parents")
    parent = relationship("Parent", back_populates="children")

class Parent(Base):
    __tablename__ = 'left'
    id = Column(Integer, primary_key=True)
    children = relationship("Association", back_populates="parent")

class Child(Base):
    __tablename__ = 'right'
    id = Column(Integer, primary_key=True)
    parents = relationship("Association", back_populates="child")

parent = Parent(children=[Child()])
```

Running that code with SQLAlchemy version 1.2.11 throws this exception:

```
lars$ venv/bin/python test.py
Traceback (most recent call last):
  File "test.py", line 26, in <module>
    parent = Parent(children=[Child()])
  File "<string>", line 4, in __init__
  File "/Users/lars/coding/sqlalchemy_association_object_test/venv/lib/python3.7/site-packages/sqlalchemy/orm/state.py", line 417, in _initialize_instance
    manager.dispatch.init_failure(self, args, kwargs)
  File "/Users/lars/coding/sqlalchemy_association_object_test/venv/lib/python3.7/site-packages/sqlalchemy/util/langhelpers.py", line 66, in __exit__
    compat.reraise(exc_type, exc_value, exc_tb)
  File "/Users/lars/coding/sqlalchemy_association_object_test/venv/lib/python3.7/site-packages/sqlalchemy/util/compat.py", line 249, in reraise
    raise value
  File "/Users/lars/coding/sqlalchemy_association_object_test/venv/lib/python3.7/site-packages/sqlalchemy/orm/state.py", line 414, in _initialize_instance
    return manager.original_init(*mixed[1:], **kwargs)
  File "/Users/lars/coding/sqlalchemy_association_object_test/venv/lib/python3.7/site-packages/sqlalchemy/ext/declarative/base.py", line 737, in _declarative_constructor
    setattr(self, k, kwargs[k])
  File "/Users/lars/coding/sqlalchemy_association_object_test/venv/lib/python3.7/site-packages/sqlalchemy/orm/attributes.py", line 229, in __set__
    instance_dict(instance), value, None)
  File "/Users/lars/coding/sqlalchemy_association_object_test/venv/lib/python3.7/site-packages/sqlalchemy/orm/attributes.py", line 1077, in set
    initiator=evt)
  File "/Users/lars/coding/sqlalchemy_association_object_test/venv/lib/python3.7/site-packages/sqlalchemy/orm/collections.py", line 762, in bulk_replace
    appender(member, _sa_initiator=initiator)
  File "/Users/lars/coding/sqlalchemy_association_object_test/venv/lib/python3.7/site-packages/sqlalchemy/orm/collections.py", line 1044, in append
    item = __set(self, item, _sa_initiator)
  File "/Users/lars/coding/sqlalchemy_association_object_test/venv/lib/python3.7/site-packages/sqlalchemy/orm/collections.py", line 1016, in __set
    item = executor.fire_append_event(item, _sa_initiator)
  File "/Users/lars/coding/sqlalchemy_association_object_test/venv/lib/python3.7/site-packages/sqlalchemy/orm/collections.py", line 680, in fire_append_event
    item, initiator)
  File "/Users/lars/coding/sqlalchemy_association_object_test/venv/lib/python3.7/site-packages/sqlalchemy/orm/attributes.py", line 943, in fire_append_event
    state, value, initiator or self._append_token)
  File "/Users/lars/coding/sqlalchemy_association_object_test/venv/lib/python3.7/site-packages/sqlalchemy/orm/attributes.py", line 1210, in emit_backref_from_collection_append_event
    child_impl = child_state.manager[key].impl
KeyError: 'parent'
```