foreign key exist 
Questions go on stackoverflow. http://docs.peewee-orm.com/en/latest/peewee/contributing.html#questions
hi, 
would like check if foreign key exist do something.
```
  if self.printer_type:
        result['type_id'] = self.printer_type.id
        result['type_name'] = self.printer_type.name
```
like this. I think the example doesn't work