Organizing museum artwork
Django's philosophy is Thin views, Fat models. This may be true for small applications, as application grows, it becomes extremely complex to manage all business actions in model layer.

One of the suggested approach is to have a separate layer to handle business actions. This can be similar to view, name it orchestrator or business_actions. 

The responsibility of the layer is to do one business action which can touch multiple models. Say, register a new client means creating new user, new client, send notification, sync to external system. Having all these in multiple models is cumbersome.

The business action layer can be modelled as class or simple function which takes data and does all the relevant work. 

Here is an example todo application which is designed to address these concerns.