Interactor

Getting Started

Add Interactor to your Gemfile and bundle install.

gem "interactor", "~> 3.0"

What is an Interactor?

An interactor is a simple, single-purpose object.

Interactors are used to encapsulate your application's business logic. Each interactor represents one thing that your application does.

Context

An interactor is given a context. The context contains everything the interactor needs to do its work.

When an interactor does its single purpose, it affects its given context.

Adding to the Context

As an interactor runs it can add information to the context.

context.user = user

Failing the Context

When something goes wrong in your interactor, you can flag the context as failed.

context.fail!

When given a hash argument, the fail! method can also update the context. The following are equivalent:

context.error = "Boom!"
context.fail!

context.fail!(error: "Boom!")

You can ask a context if it's a failure:

context.failure? # => false
context.fail!
context.failure? # => true

or if it's a success.

context.success? # => true
context.fail!
context.success? # => false

Dealing with Failure

context.fail! always throws an exception of type Interactor::Failure.

Normally, however, these exceptions are not seen. In the recommended usage, the controller invokes the interactor using the class method call, then checks the success? method of the context.

This works because the call class method swallows exceptions. When unit testing an interactor, if calling custom business logic methods directly and bypassing call, be aware that fail! will generate such exceptions.

See Interactors in the Controller, below, for the recommended usage of call and success?.

Hooks

Before Hooks

Sometimes an interactor needs to prepare its context before the interactor is even run. This can be done with before hooks on the interactor.

before do
  context.emails_sent = 0
end

A symbol argument can also be given, rather than a block.

before :zero_emails_sent

def zero_emails_sent
  context.emails_sent = 0
end

After Hooks

Interactors can also perform teardown operations after the interactor instance is run.

after do
  context.user.reload
end

NB: After hooks are only run on success. If the fail! method is called, the interactor's after hooks are not run.

Around Hooks

You can also define around hooks in the same way as before or after hooks, using either a block or a symbol method name. The difference is that an around block or method accepts a single argument. Invoking the call method on that argument will continue invocation of the interactor. For example, with a block:

around do |interactor|
  context.start_time = Time.now
  interactor.call
  context.finish_time = Time.now
end

With a method:

around :time_execution

def time_execution(interactor)
  context.start_time = Time.now
  interactor.call
  context.finish_time = Time.now
end

NB: If the fail! method is called, all of the interactor's around hooks cease execution, and no code after interactor.call will be run.

Hook Sequence

Before hooks are invoked in the order in which they were defined while after hooks are invoked in the opposite order. Around hooks are invoked outside of any defined before and after hooks. For example:

around do |interactor|
  puts "around before 1"
  interactor.call
  puts "around after 1"
end

around do |interactor|
  puts "around before 2"
  interactor.call
  puts "around after 2"
end

before do
  puts "before 1"
end

before do
  puts "before 2"
end

after do
  puts "after 1"
end

after do
  puts "after 2"
end

will output:

around before 1
around before 2
before 1
before 2
after 2
after 1
around after 2
around after 1

Interactor Concerns

An interactor can define multiple before/after hooks, allowing common hooks to be extracted into interactor concerns.

module InteractorTimer
  extend ActiveSupport::Concern

  included do
    around do |interactor|
      context.start_time = Time.now
      interactor.call
      context.finish_time = Time.now
    end
  end
end

An Example Interactor

Your application could use an interactor to authenticate a user.

class AuthenticateUser
  include Interactor

  def call
    if user = User.authenticate(context.email, context.password)
      context.user = user
      context.token = user.secret_token
    else
      context.fail!(message: "authenticate_user.failure")
    end
  end
end

To define an interactor, simply create a class that includes the Interactor module and give it a call instance method. The interactor can access its context from within call.

Interactors in the Controller

Most of the time, your application will use its interactors from its controllers. The following controller:

class SessionsController < ApplicationController
  def create
    if user = User.authenticate(session_params[:email], session_params[:password])
      session[:user_token] = user.secret_token
      redirect_to user
    else
      flash.now[:message] = "Please try again."
      render :new
    end
  end

  private

  def session_params
    params.require(:session).permit(:email, :password)
  end
end

can be refactored to:

class SessionsController < ApplicationController
  def create
    result = AuthenticateUser.call(session_params)

    if result.success?
      session[:user_token] = result.token
      redirect_to result.user
    else
      flash.now[:message] = t(result.message)
      render :new
    end
  end

  private

  def session_params
    params.require(:session).permit(:email, :password)
  end
end

The call class method is the proper way to invoke an interactor. The hash argument is converted to the interactor instance's context. The call instance method is invoked along with any hooks that the interactor might define. Finally, the context (along with any changes made to it) is returned.

When to Use an Interactor

Given the user authentication example, your controller may look like:

class SessionsController < ApplicationController
  def create
    result = AuthenticateUser.call(session_params)

    if result.success?
      session[:user_token] = result.token
      redirect_to result.user
    else
      flash.now[:message] = t(result.message)
      render :new
    end
  end

  private

  def session_params
    params.require(:session).permit(:email, :password)
  end
end

For such a simple use case, using an interactor can actually require more code. So why use an interactor?

Clarity

We often use interactors right off the bat for all of our destructive actions (POST, PUT and DELETE requests) and since we put our interactors in app/interactors, a glance at that directory gives any developer a quick understanding of everything the application does.

▾ app/
  ▸ controllers/
  ▸ helpers/
  ▾ interactors/
      authenticate_user.rb
      cancel_account.rb
      publish_post.rb
      register_user.rb
      remove_post.rb
  ▸ mailers/
  ▸ models/
  ▸ views/

TIP: Name your interactors after your business logic, not your implementation. CancelAccount will serve you better than DestroyUser as the account cancellation interaction takes on more responsibility in the future.

The Future™

SPOILER ALERT: Your use case won't stay so simple.

In our experience, a simple task like authenticating a user will eventually take on multiple responsibilities:

• Welcoming back a user who hadn't logged in for a while
• Prompting a user to update his or her password
• Locking out a user in the case of too many failed attempts
• Sending the lock-out email notification

The list goes on, and as that list grows, so does your controller. This is how fat controllers are born.

If instead you use an interactor right away, as responsibilities are added, your controller (and its tests) change very little or not at all. Choosing the right kind of interactor can also prevent simply shifting those added responsibilities to the interactor.

Testing Interactors

When written correctly, an interactor is easy to test because it only does one thing. Take the following interactor:

class AuthenticateUser
  include Interactor

  def call
    if user = User.authenticate(context.email, context.password)
      context.user = user
      context.token = user.secret_token
    else
      context.fail!(message: "authenticate_user.failure")
    end
  end
end

You can test just this interactor's single purpose and how it affects the context.

describe AuthenticateUser do
  subject(:context) { AuthenticateUser.call(email: "[email protected]", password: "secret") }

  describe ".call" do
    context "when given valid credentials" do
      let(:user) { double(:user, secret_token: "token") }

      before do
        allow(User).to receive(:authenticate).with("[email protected]", "secret").and_return(user)
      end

      it "succeeds" do
        expect(context).to be_a_success
      end

      it "provides the user" do
        expect(context.user).to eq(user)
      end

      it "provides the user's secret token" do
        expect(context.token).to eq("token")
      end
    end

    context "when given invalid credentials" do
      before do
        allow(User).to receive(:authenticate).with("[email protected]", "secret").and_return(nil)
      end

      it "fails" do
        expect(context).to be_a_failure
      end

      it "provides a failure message" do
        expect(context.message).to be_present
      end
    end
  end
end

We use RSpec but the same approach applies to any testing framework.

Isolation

You may notice that we stub User.authenticate in our test rather than creating users in the database. That's because our purpose in spec/interactors/authenticate_user_spec.rb is to test just the AuthenticateUser interactor. The User.authenticate method is put through its own paces in spec/models/user_spec.rb.

It's a good idea to define your own interfaces to your models. Doing so makes it easy to draw a line between which responsibilities belong to the interactor and which to the model. The User.authenticate method is a good, clear line. Imagine the interactor otherwise:

class AuthenticateUser
  include Interactor

  def call
    user = User.where(email: context.email).first

    # Yuck!
    if user && BCrypt::Password.new(user.password_digest) == context.password
      context.user = user
    else
      context.fail!(message: "authenticate_user.failure")
    end
  end
end

It would be very difficult to test this interactor in isolation and even if you did, as soon as you change your ORM or your encryption algorithm (both model concerns), your interactors (business concerns) break.

Draw clear lines.

Integration

While it's important to test your interactors in isolation, it's just as important to write good integration or acceptance tests.

One of the pitfalls of testing in isolation is that when you stub a method, you could be hiding the fact that the method is broken, has changed or doesn't even exist.

When you write full-stack tests that tie all of the pieces together, you can be sure that your application's individual pieces are working together as expected. That becomes even more important when you add a new layer to your code like interactors.

TIP: If you track your test coverage, try for 100% coverage before integrations tests. Then keep writing integration tests until you sleep well at night.

Controllers

One of the advantages of using interactors is how much they simplify controllers and their tests. Because you're testing your interactors thoroughly in isolation as well as in integration tests (right?), you can remove your business logic from your controller tests.

class SessionsController < ApplicationController
  def create
    result = AuthenticateUser.call(session_params)

    if result.success?
      session[:user_token] = result.token
      redirect_to result.user
    else
      flash.now[:message] = t(result.message)
      render :new
    end
  end

  private

  def session_params
    params.require(:session).permit(:email, :password)
  end
end

describe SessionsController do
  describe "#create" do
    before do
      expect(AuthenticateUser).to receive(:call).once.with(email: "[email protected]", password: "secret").and_return(context)
    end

    context "when successful" do
      let(:user) { double(:user, id: 1) }
      let(:context) { double(:context, success?: true, user: user, token: "token") }

      it "saves the user's secret token in the session" do
        expect {
          post :create, session: { email: "[email protected]", password: "secret" }
        }.to change {
          session[:user_token]
        }.from(nil).to("token")
      end

      it "redirects to the homepage" do
        response = post :create, session: { email: "[email protected]", password: "secret" }

        expect(response).to redirect_to(user_path(user))
      end
    end

    context "when unsuccessful" do
      let(:context) { double(:context, success?: false, message: "message") }

      it "sets a flash message" do
        expect {
          post :create, session: { email: "[email protected]", password: "secret" }
        }.to change {
          flash[:message]
        }.from(nil).to(I18n.translate("message"))
      end

      it "renders the login form" do
        response = post :create, session: { email: "[email protected]", password: "secret" }

        expect(response).to render_template(:new)
      end
    end
  end
end

This controller test will have to change very little during the life of the application because all of the magic happens in the interactor.

Rails

We love Rails, and we use Interactor with Rails. We put our interactors in app/interactors and we name them as verbs:

• AddProductToCart
• AuthenticateUser
• PlaceOrder
• RegisterUser
• RemoveProductFromCart

See: Interactor Rails

Contributions

Interactor is open source and contributions from the community are encouraged! No contribution is too small.

See Interactor's contribution guidelines for more information.

Thank You

A very special thank you to Attila Domokos for his fantastic work on LightService. Interactor is inspired heavily by the concepts put to code by Attila.

Interactor was born from a desire for a slightly simplified interface. We understand that this is a matter of personal preference, so please take a look at LightService as well!