`unique_together` constraint not working

I might be doing something wrong, but if the following example code is executed:

```
Attribute.objects.create(name='Height', datatype=Attribute.TYPE_TEXT)
```

There is no `content_type`. Furthermore, it can be executed multiple times resulting in several attributes, and judging from `unique_together = ('site', 'content_type', 'slug')`, perhaps it is not desirable?