How to get foreign key column itself?
You can use `Pet._data['owner']`.
 Thank you

Example for this:
class Person(Model):
    name = CharField()
    birthday = DateField()

class Pet(Model):
    owner = ForeignKeyField(Person, related_name='pets')
    name = CharField()

I want to get pet.owner_id, but I can only use pet.owner.id, which will execute a sql query extraly.
