In Praxis, controllers behave much the same as they do in other MVC-based web frameworks — they expose actions which receive requests from API consumers, operate on resources to service those requests, and return the appropriate responses. In short, controllers are the glue which connects actions and their responses to application business logic.

Praxis controllers differ from some other frameworks in that they:

  • are plain Ruby classes that happen to include the Praxis::Controller module.
  • execute an action by calling a method. Action methods accept Ruby named parameters corresponding to the attribute names of the resource definition.

By using plain Ruby classes, Praxis allows you to use the full power of Ruby. You are not limited in your inheritance options, and your controller code can be well-isolated and easily tested.

Implementing a Controller

To implement a controller in Praxis, include the Praxis::Controller module in your controller class and indicate which of your Resource Definitions it implements by using the implements stanza.

class Posts
  include Praxis::Controller

  implements PostsDefinition

  # Controller code...

Including the Praxis::Controller module enhances the class with methods such as implements, before, and after.

The implements method is used to connect a controller with its ResourceDefinition. Technically speaking there is nothing that prevents the same class from being both a Resource Definition and its Controller, implementing itself. Despite that being feasible due to the modularity that Praxis provides, we discourage it in order to keep “definitions” logically separate from “runtime code”.

Once set, you can retrieve the ResourceDefinition for a controller with the definition method, which is defined on both the class and instance.

Implementing an Action

A controller action is an instance method defined on a controller class. The method’s name must match an action defined in the controller’s resource definition.

For example, this resource definition class for Posts defines two actions — :index and :show:

class PostsDefinition
  include Praxis::ResourceDefinition

  media_type Post
  version '1.0'

  action :index do
    routing { get '' }
    description 'Fetch all blog posts'

  action :show do
    routing { get '/:id' }
    description 'Fetch an individual blog post'
    params do
      attribute :id, Integer, required: true
      attribute :token, String
      attribute :allow_deleted, Attributor::Boolean
      attribute :extended_info, Attributor::Boolean

The controller implementing this resource definition must have instance methods named index and show which accept the argument names described by the params block from the resource definition.

class Posts
  include Praxis::Controller

  implements PostsDefinition

  def index
    # empty method signature: the index action defines no parameters

  def show(id:, token:, **other_params)
    # four parameters defined matching the names of the arguments
    # Note that ruby allows is to unpack only the names we care about
    # and leave the rest tucked away in the other_params hash

Note that the index action has no parameters defined in its resource definition so the method accepts no arguments.

On the other hand, the show action has four parameters defined in its resource definition, so it can explicitly declare them as named method arguments. Ruby gives you great flexibility in declaring named parameters with the splat operator. It is up to the developer to choose how many explicit arguments to list, and how many to tuck away inside an other_params hash. In this case, the developer decided that id and token are important enough to use as direct variables in the controller, while pushing the allowed_deleted and extended_info into the other_params hash. Having this flexibility is great for dealing with large number of parameters while keeping your controller code tidy.

In addition to using named arguments for incoming parameters, Praxis will also ensure their values match the types that you’ve specified in the Resource Definition. Accessing the id variable within the show method will always get you an Integer.

Retrieving Headers and Payload Data

The Praxis::Controller module provides a request accessor which can be used to retrieve the incoming headers and the payload data. The information under these methods is type-curated much like parameter definitions. They are accessible through methods matching your attribute names, and they will always return values matching the type of your attribute (possibly coercing them if necessary).

You can also test if a value exists (or has been assigned by a default option) for an attribute with the key? method. This is useful in those cases where there is an important distinction between a user-provided nil value and the user simply not providing a value, as there is in “PATCH” requests.

For completeness, the request object also gives you access to your params in the same way, even though you already get them passed in as named arguments.

Here’s an example of how to access these methods from a controller action:

def show(id:, token:, **other_params)
  accept = request.headers.accept # Retrieve 'Accept' header
  if request.payload.key?(:view)  # whether a value was specified for 'view'
    view = request.payload.view   # Retrieve a 'view' parameter from the payload
  id ==         # id argument will be the same as

Returning a Response

Every controller action is responsible for returning a Response object (an instance of a Praxis::Response-derived class) with the right headers, status code and body. Praxis handles the work of delivering your responses to clients.

Instead of having to create a Response object every time, each controller instance comes with a pre-set response accessor containing an instance of the Default response which is 200 OK unless you modify it. Your controller may use and modify the default response or substitute it with another. Or it could ignore the default accessor and return its own Response objects. Here’s an example of one way to use the default accessor:

def show(id:, token:, **other_params)
  response.headers['Content-Type'] = 'text/plain'
  response.body = "This is a simple body"

There is another way to use the default response. If your action returns a string, Praxis will call response.body for you and implicitly use that response.

def show(id:, token:, **other_params)
  response.headers['Content-Type'] = 'text/plain'
  "This is a simple body"

Request Life Cycle Callbacks

Praxis provides a way to register one or more callbacks before or after named stages in the request life cycle. This is done using the before and after methods which take zero or more params and a block for Praxis to execute.

To execute a callback before the show action runs, you could add:

before actions: [:show] do
  puts "before action"

To execute a callback before the validate stage of the request cycle, but only when the action is index, you could add:

before :validate, actions: [:index] do |controller|
  puts "About to validate params/headers/payload for action: #{}"

The block receives the instance of your controller, which you can use to access all of the controller’s properties, including the request, the response, any actions, etc.

For a complete discussion of what stages are available for use in your callbacks, as well as how to use them, please refer to the Request Life Cycle documentation.