Generally the suggestion is to go about using Fat Models and Thin Controllers. So business logic in your model. It makes it far easier to make your code re-useable.
More info here: http://www.yiiframework.com/doc/guide/1.1/en/basics.best-practices
If you've got a lot of custom logic, you could potentially have an "asset" layer of additional models that handled your DB models. Depends on your specific system though … I'm finding I do use CFormModel
objects this way at times to map from a form with a bunch of different models to the models as needed.