Can't use Model class as field value
It isn't calling `db_value()` because peewee sees that you're using a model class as a value, which is a recognized AST type. The issue that originated this fix is #2131. Thanks for reporting, this is fixed in master now. Thank you!  I just wanted to drop a note saying how much I've enjoyed using peewee and how appreciative I've been have your responsiveness to issues.
I have a custom field that stores the import path to a Python class.  The `db_value` takes the class object and coverts it to the import path string and the `python_value` takes the string and imports the object giving me back the class.  However, as of peewee 3.13.2 this no longer works and instead gives me `peewee.InternalError: (1054, "Unknown column 't1' in 'field list'")` when I try to save.

Field:
```python
class PythonObjectField(CharField):
    """
    A custom class for storing a Python object as a reference.
    """

    def db_value(self, value):
        if value is None:
            return None
        elif isinstance(value, str):
            return value

        return get_object_path(value)

    def python_value(self, value):
        if value is None:
            return None

        return import_object(value)
```

The usage would be `Model.field = SomeOtherModel`

[Full example script](https://github.com/coleifer/peewee/files/4652313/peewee_example.txt)

When debugging this I found that this affects my `Model` derived classes but not a simple one that only extends `object`.  Additionally, I found that `db_value` is never called.

