Support for model mixins?
@foxx, try with:

```
class BaseModelMixin(Model):
```
 @yohanboniface has it right.
 Additional nit, you don't need `unique=True` for a primary key.
 Ahh okay, so the mixins have to have subclass `Model`, I'm guessing due to metaclass magic of some sort. Looks to be working now, thanks both :) (also will remove `unique=True`, ty)

Currently it's not possible to do the following;

``` python
class BaseModelMixin(object):
    id = UUIDField(default=uuid1, primary_key=True, unique=True)

class BaseModel(BaseModelMixin, Model):
    pass

class User(BaseModel):
    pass
```

It's surprising behaviour that I can't construct models from a set of mixins, are there any plans to support this? If not, could you give some insight into why?
