ForeignKeyField(..., primary_key=True)
Just add a `property` or whatever:

```python
class Rone(Model):
    @property
    def r2(self):
        return self.rtwo.get()
```
When defining a ForeignKeyField who is also the Primary key for the table, It would make sense to have the backref to be a reference to the record and not a select instance as there can be only one record it is referring to.

class Rone(BaseModel):
    xxx = CharField(primary_key=True)
    name = CharField(null=True)

class Rtwo(BaseModel):
    xxx = ForeignKeyField(Rone, db_column='xxx', backref='rtwo', primary_key=True)
    num = IntegerField(null=True)

Works as expected:
rtwo=Rtwo().get(xxx='monkey')
print(rtwo.xxx.name)

Works differently than I would like:
rone=Rone().get(xxx='monkey')
print(rone.rtwo.num) # doesn't work
print(rone.rtwo.get().num) # works
