[Sqlalchemy-tickets] Issue #3857: Attribute history is (sometimes) not updated unless model is queried for (zzzeek/sqlalchemy)

[David P. <iss...@bi...>]

New issue 3857: Attribute history is (sometimes) not updated unless model is queried for
https://bitbucket.org/zzzeek/sqlalchemy/issues/3857/attribute-history-is-sometimes-not-updated

David Pärsson:

Consider this example:
```python
model = Model(string='first value')

session.add(model)
session.commit()

model.string = 'second value'

session.add(model)
session.commit()

model = session.query(Model).one()
model.string = 'third value'

session.add(model)
session.commit()
```

The `history.deleted` for this attribute is only updated after the model has been queried for:

```
$ python3 repro.py 
Instance is new. History for attribute "string" is: History(added=['first value'], unchanged=(), deleted=())
Instance is dirty. History for attribute "string" is: History(added=['second value'], unchanged=(), deleted=())
Instance is dirty. History for attribute "string" is: History(added=['third value'], unchanged=(), deleted=['second value'])
```

I would expect `deleted` to be `['first value']` for the first row starting with `Instance is dirty.`.

Please se the attached file for the full, runnable example.