[Sqlalchemy-tickets] Issue #3145: AttributeError: 'NoneType' object has no attribute '_sa_appender' (zzzeek/sqlalchemy)

[Dobes V. <iss...@bi...>]

New issue 3145: AttributeError: 'NoneType' object has no attribute '_sa_appender'
https://bitbucket.org/zzzeek/sqlalchemy/issue/3145/attributeerror-nonetype-object-has-no

Dobes Vandermeer:

Under certain circumstances the above error can occur.  It's a bit hard to explain, but I will try.

In our case:

1. An entity joins to itself in a parent/child relation
2. An instance has its parent relation set to itself
3. A collection in the parent entity is eagerly loaded as part of a query
4. The properties of the entity are ordered such that the collection is loaded after the parent

This triggers a sequence of events in which:

1. The child is loaded
2. The parent is loaded (which is actually the child) as part of loading properties
3. The parent initializes the collection for eager loading
4. The child continues setting up its properties, setting the same collection to the default (lazy loaded) collection
5. The collection set up in the parent to collect the eagerly loaded entities is garbage collected
6. When a member of the collection is loaded on a subsequent row, there is a crash because the underlying collection was garbage collected

The stack trace is as follows:


```
#!python

Traceback (most recent call last):
  File "/home/dobes/sqlalchemy/test/orm/test_temp.py", line 62, in test_bug
    res = session.query(A).join(A.b).join(b_parent, b_parent.b_id == B.parent_id).join(b_parent.z).filter(BC.value>0).options(joinedload('b').joinedload('parent').joinedload('z').joinedload('c')).all()
  File "/home/dobes/sqlalchemy/test/../lib/sqlalchemy/orm/query.py", line 2300, in all
    return list(self)
  File "/home/dobes/sqlalchemy/test/../lib/sqlalchemy/orm/loading.py", line 73, in instances
    rows = [process[0](row, None) for row in fetch]
  File "/home/dobes/sqlalchemy/test/../lib/sqlalchemy/orm/loading.py", line 455, in _instance
    populate_state(state, dict_, row, isnew, only_load_props)
  File "/home/dobes/sqlalchemy/test/../lib/sqlalchemy/orm/loading.py", line 305, in populate_state
    populator(state, dict_, row)
  File "/home/dobes/sqlalchemy/test/../lib/sqlalchemy/orm/strategies.py", line 1483, in load_scalar_from_joined_existing_row
    existing = _instance(row, None)
  File "/home/dobes/sqlalchemy/test/../lib/sqlalchemy/orm/loading.py", line 484, in _instance
    populate_state(state, dict_, row, isnew, attrs)
  File "/home/dobes/sqlalchemy/test/../lib/sqlalchemy/orm/loading.py", line 309, in populate_state
    populator(state, dict_, row)
  File "/home/dobes/sqlalchemy/test/../lib/sqlalchemy/orm/strategies.py", line 1483, in load_scalar_from_joined_existing_row
    existing = _instance(row, None)
  File "/home/dobes/sqlalchemy/test/../lib/sqlalchemy/orm/loading.py", line 484, in _instance
    populate_state(state, dict_, row, isnew, attrs)
  File "/home/dobes/sqlalchemy/test/../lib/sqlalchemy/orm/loading.py", line 309, in populate_state
    populator(state, dict_, row)
  File "/home/dobes/sqlalchemy/test/../lib/sqlalchemy/orm/strategies.py", line 1465, in load_collection_from_joined_existing_row
    _instance(row, result_list)
  File "/home/dobes/sqlalchemy/test/../lib/sqlalchemy/orm/loading.py", line 503, in _instance
    result.append(instance)
  File "/home/dobes/sqlalchemy/test/../lib/sqlalchemy/util/_collections.py", line 757, in append
    self._data_appender(item)
  File "/home/dobes/sqlalchemy/test/../lib/sqlalchemy/orm/collections.py", line 657, in append_without_event
    self._data()._sa_appender(item, _sa_initiator=False)
AttributeError: 'NoneType' object has no attribute '_sa_appender'

```

I've attached a test case that reproduces the issue.  Note that because the order of properties is significant, I did a bit of a hack to sort the properties.