July 23, 2019

A presenter pattern for Rails controllers

Rails controllers have a trick to pass data to the view: all instance variables are copied over after an action method is executed. This is at odds with good Ruby object design and causes practical problems. It becomes awkward to refactor controller methods without exposing unwanted state to the view — a private method that sets an instance variable has side effects elsewhere in the code. Controllers have the strange task of mutating their inner state as their public behavior.

I’d have guessed that Object#instance_variable_get and #instance_variable_set were private methods. Not so. Apparently it is “public” API for you to mess with any Ruby object’s internals.

How many Ruby classes have you needed to test by examining their instance variables? I don’t think I’ve done this anywhere, except in controller tests, where it’s the norm.

It is true that the trick is aesthetically pleasing for simple controllers and for when you need to demo Rails. Take a standard Rails controller with one simple action:

And now, in the corresponding view, the @post is available:

For simple controllers, you might think of the view as an extension of the controller. It has access to private internals. The template is rendered as if it were just another method on the controller.

But in the MVC pattern, views and controllers have different concerns. A controller generally manipulates model objects to interact with the database; a view should not. Controllers send email, spawn background jobs, handle validation failure, catch exceptions. Views do not.

Using ActiveRecord objects in the view seems like a good thing to avoid if possible. These are generally the most powerful objects in a Rails app. They have an enormous API and many methods query the database. One of our models at Freebird has 619 instance methods (not counting those from Object) — most of those are added by ActiveRecord. It’s probably a bad idea for a view, responsible for producing HTML, to execute SQL queries.

Additionally, objects used in a view often need methods specific to formatting content for the user. Adding these to ActiveRecord classes makes their large interface even wider.

At Freebird, we’re moving to using Presenter objects that wrap ActiveRecord objects, exposing only the methods that the view needs and adding view-specific logic. But we found it was a little awkward to introduce them into standard controllers. Consider this code:

Even in this simple show action, there is some awkwardness with naming. @post in the view is now a PostPresenter — good, this is what we want. Unfortunately, now the controller must distinguish between model and presenter objects, and with the @post name reserved for the presenter, our model variables get an awkward name like post_model. We have to rename and rearrange our controller’s instance variables because their names are used in the view; something about this feels wrong.

With a few more actions, the situation gets worse:

An alternative is to overwrite the @post variable for the view, e.g., @post = PostPresenter.new(@post). But this is error-prone, confusing and — were we using Sorbet — a type error.

We’ve factored out common logic into require_post, but in doing so, our views now have access to the @post_model instance variable. And we now have two kinds of instance variables, one that is meant to be passed and used in the view, and one that is not. This makes the intention of the code harder to follow, especially in more complex controllers.

Thankfully, this is easy to address. The public method used to populate the view with instance variables is AbstractController#view_assigns. It builds a hash from all the instance variables in the controller object. It is not a complicated method:

Interestingly, in Merb, views were just methods on controllers, meaning the apparent sharing of state was structural. I think Rails got it right here.

Merb’s “proof that everything belongs in one class to begin with” was that it was more performant.

Controllers inherit from AbstractController, so we can override view_assigns to do whatever we want. Let’s have it simply return a hash that is set in a new method called present:

Now we can include the Presenters module in our controller, and our instance variables will not be passed to the views. Instead, we explicitly assign variables to the view with present:

The awkwardness of controller instance variables is solved — they can simply be used to share instance state, as they are meant to do. We can refactor controller code without worrying what will end up in a view.

We can clean up this code a little more. If we assume that a Presenter is always instantiated in the same way — a ModelNamePresenter accepts a ModelName object as a single initialization parameter — we can instantiate the presenter using the class of the object passed in:

Calling present(post: @post) now inspects the class name of @post, finds the corresponding Presenter class and instantiates a presenter with the @post object passed to the constructor. The final controller looks like this:

The present method will fail with a uninitialized constant exception if an appropriate Presenter class is not found. In this way we enforce some consistency in the view: instance variables must be presenter objects.

We have split apart the shared scope between the controller and the view, with the present method providing the interface between them. A developer working in controller code can be confident instance variables incidentally used for refactoring actions won’t affect view code. A front end developer knows exactly which variables were meant for the view. If nothing else, there is a self-documenting nature to present that the standard Rails controller lacks.

Freebird has extracted a simple library for setting this up in Rails controllers, as well as providing a base Presenter class with some conveniences. Check it out, it’s called Livery. It does a bit more than the PostsController example here, but the basic idea is the same. For instance, our implementation of present handles passing in presenter objects directly, objects with module namespaces, and collections.