[Sqlalchemy-tickets] Issue #3481: AttributeError: 'NoneType' object has no attribute '_getter' in sqlalchemy.engine.result (zzzeek/sqlalchemy)

[Hanno S. <iss...@bi...>]

New issue 3481: AttributeError: 'NoneType' object has no attribute '_getter' in sqlalchemy.engine.result
https://bitbucket.org/zzzeek/sqlalchemy/issues/3481/attributeerror-nonetype-object-has-no

Hanno Schlichting:

Got a Sentry report from a live site today, using SQLAlchemy 1.0.6, PyMySQL 0.6.3 against a Amazon RDS MySQL 5.6 instance.

This happened just once, in a code path that is frequently executed, so it must be some kind of weird state it got in, though I don't know what state that is.

Looking through the issue tracker, this might be related to a new 1.0 feature via issue #3175.

Full traceback:

```
#!python

Stacktrace (most recent call last):

  <removed app code specific stack frames>

  File "sqlalchemy/orm/query.py", line 2445, in first
    ret = list(self[0:1])
  File "sqlalchemy/orm/query.py", line 2281, in __getitem__
    return list(res)
  File "sqlalchemy/orm/loading.py", line 84, in instances
    util.raise_from_cause(err)
  File "sqlalchemy/util/compat.py", line 199, in raise_from_cause
    reraise(type(exception), exception, tb=exc_tb)
  File "sqlalchemy/orm/loading.py", line 51, in instances
    for query_entity in query._entities
  File "sqlalchemy/orm/query.py", line 3312, in row_processor
    polymorphic_discriminator=self._polymorphic_discriminator
  File "sqlalchemy/orm/loading.py", line 313, in _instance_processor
    getter = result._getter(col)
  File "sqlalchemy/engine/result.py", line 495, in _getter
    return self._metadata._getter(key)

AttributeError: 'NoneType' object has no attribute
```

The query going into this is a simple:
```
#!python

session.query(SomeModel).filter(SomeModel.pk_column == 'somevalue')
.first()
```

In the last stackframe, the locals are:
```
#!python

key = Column('some_column', INTEGER(), table=<some_table>)
self = <sqlalchemy.engine.result.ResultProxy object at 0x4aa7e50>
```

It looks to me like `ResultProxy._metadata` ends up being `None`, even after `_init_metadata` is called. I see a bunch of `if self._metadata is None` cases in the other functions, so maybe this just needs one of those protections as well.