I am struggling with how to understand the correct usage of models. Currently i use the inheritance of Db\_Table directly and declare all the business logic there. I know it's not correct way to do this. 

One solution would be to use Doctrine ORM, but this requires learning curve and all the current components what i use needs to be rewritten paginator and auth. Also Doctrine1 adds a another dozen classes which need to be loaded.

So the current cleanest implementation what i have seen is to use the Data Mapper classes between the so called model and DbTabel. I haven't yet implemented this as it seems to head writing another ORM. But example could be something this: SQL table User

* create class with setters, getters, business logic here */model/User.php*
* data mapper */model/mapper/UserMapper.php*, the funcionality is basically writing all the update, save actions in here.
* the data source */model/DbTable/User.php* extends the Db\_Table\_Abstract

Problems are with relationships between other models.