models.py

class Users(Base):
    uid = PrimaryKeyField()
    name = CharField(null=True)
    email = CharField(unique=True, null=True)
    country = CharField(null=True)
    interests = TextField(null=True)
    reference = TextField(null=True)
    scope = TextField(null=True)


class States(Base):
    uid = ForeignKeyField(Users, related_name='states', primary_key=True)
    state = CharField()


registration.py

def save_email(uid, email):
    user = States.select().join(Users).where(Users.uid == uid)[0]
    user.email = email
    user.state = COUNTRY
    user.save()
    return user

test_registration.py

def test_enter_email():
    user = save_email(1, 'qwerty@gmail.com')
    assert user.email is not None
    assert user.email == 'qwerty@gmail.com'
    assert user.state == 'country'