Model cache being inconsistent when the value is set by id
Thanks!

Project have a ForeignKeyField referencing Client
The output is from my projects iPython shell

```
In [1]: p = Project.get(id=581)

In [2]: p.client
Out[2]: <models.Client at 0x1f6d790>

In [3]: p.client.id
Out[3]: 115

In [4]: p.client = 123

In [5]: p.client.id
Out[5]: 115
```
