• Controllers 
  • can call only models
  • can call only services
  • cannot call helper - anti-pattern
  • cannot call rake task - anti-pattern
  • cannot call migrations - anti-pattern
• Models 
  • can be used only in controller
  • can be used only in services
  • can be used in rake task
  • can be used in migrations
  • maybe in other models
  • cannot be used in views - anti-pattern
  • cannot be used in helpers - anti-pattern
• Views 
  • cannot call model - anti-pattern
  • cannot call services - anti-pattern
  • can call only helpers
• Helper 
  • cannot be used in model - anti-pattern
  • cannot be used in controller - anti-pattern
  • only for views

More detailed

• Controllers 
  • Can call models and services, but it's generally recommended to keep business logic out of controllers and instead use service objects to encapsulate complex logic.
  • Should not call helpers, rake tasks, or migrations as these are outside the scope of the controller's responsibilities.
• Models 
  • Can be used in controllers and services, but it's generally recommended to keep controller logic separate from model logic and instead use services to encapsulate business logic.
  • Can be used in rake tasks and migrations, as these are both part of the Rails framework and can access the model layer directly.
  • Should not be used in views or helpers, as this can create tight coupling between the view and the model layer.
• Services 
  • Can call models and other services to implement business logic, but should not contain view-specific logic or interact directly with the controller layer.
  • Can be used in controllers and other services, but should not be used in views or helpers.
• Views 
  • Should not call models, services, or controllers directly, as this can lead to a mix of responsibilities and make the code harder to maintain.
  • Can call helper methods to encapsulate view-specific logic, but these helpers should not contain business logic or interact directly with the model or controller layers.
• Helpers 
  • Should only be used for view-specific logic and should not contain business logic or interact directly with the model or controller layers.
  • Cannot be used in models or controllers, as this can create tight coupling and make the code harder to maintain.