Are you sure?
Decorators decorate your class with new set of functionality.
What are your decorators doing? Adding a few rendering specific methods to a class to help with rendering? Perhaps you should consider Presenters. But better: how will it scale, can it be grouped, will it really add the simplification. Be wary of too quick branching off functionality to decorators. Most cases I've seen them were overly architectured, and they didn't bring much value.
One might consider using Concerns or mixins as an alternative. The disadvantage here is that your main object gets more public methods, but I consider it as a feature after having experienced too much potentially reusable functionality grouped arbitrarily away in other presenter / decorator classes.
So I wrote a few short articles on when to use FormObjects and Jobs and ServiceObjects. The question is of course "it depends", but the leading principle I have is keep it simple. That being said, for inspiration, some suggestion for different layers to manage the application complexity from Vladimir Dementyev's talk on Railsconf:
Presentation
- Controllers (standard Rails)
- Channels (standard Rails)
- Views (standard Rails)
- Presenters
- Form Objects
- Filter Objects
Application
- Authorization Policies
- Jobs (standard Rails)
- Event listeners
- Interactors
- Deliveries
- Notifiers
- Mailers (s…
Whenever your model gets too heavy?
The easiest way to clean up your classes might be to create smaller, more concise methods. The next easiest way of tiding up your models is moving stuff to modules (whether they are 'Concerns' or not). Modules can then be included in the final classes. It will lead to a crowded list of methods exposed on these classes, for which alternative solutions exist (Presenters, Decorators), but if you shield off private methods nicely and have a consistent way of naming things, I wouldn't be too concerned about that. Note that having many modules used in only a single class might be a code smell: perhaps you're trying to do too much with that single class.
Concerns or Modules?
When you're using Rails, you can make use of Concerns. They offer a few advantages over traditional modules, so use it whenever you're bothering recreating the same behaviour using plain old ruby Modules. I prefer consistency, so if you've adopted Concerns, use con…
When necessary.
It depends. By default I would advise against them; not creating Form objects to receive and validate data that could be validated by the Model directly. Even when you have a few nested attributes that belong to the main model modified, I would advise against Form objects. Keep It Simple.
But… sometimes you have more complex forms that don't fit the database-mirroring ActiveRecord model as nicely.
Always.
When you are able to do stuff async (not blocking the web-request), make it async. It will also reduce the need for a category of Service-objects. Worker or Job objects can often be called inline if desired.
Sidenote: I personally prefer the "Job" object name, a Job that needs to be performed. Worker is a name that was popularised by Sidekiq, but Sidekiq moved to Jobs as well.
Never.
There is of course never an absolute answer to stuff but if you are running it in a background job anyway have you considered directly writing it in a Worker or Job-object? Note that you can always run jobs async when needed.
My main objection against service objects is that all too often they are ill defined as a category. So while having fat controllers or fat models may be a bad thing, just creating a bunch of somewhat arbitrary 'Services' is not making the code more manageable.
When considering adding a 'services' directory to your app, try to think of what class of problems you want to tackle. And when in doubt, just keep messing around with the somewhat fatter models & controllers.