Ordering in 2.0.4
In the Meta class in v1, ordering is done by

``` python
class Meta:
    ordering = (('col1', 'desc'), ('col2', 'asc')
```

In version 2, this has changed to:

``` python
class Meta:
    order_by = ('-col1', 'col2') # I think I have my signs right here...
```

That's not reflected in the docs in https://peewee.readthedocs.org/en/latest/peewee/upgrading.html.
https://peewee.readthedocs.org/en/latest/peewee/models.html hints at it, but the example still shows the old `ordering` instead of `order_by` which throws an error.
