Side Effects

Side effects scenarios come up often. What if we want to send an email notification every time a Comment is created?

It’s important to note that there are three overall categories of side effects, and each requires a different solution:

• Side effects internal to the Model: For example, setting a published_at attribute.
• Side effects that should only occur on a specific request: For example, only send an email update if we’re creating a Post for the first time, at the /posts endpoint.
• Side effects that should occur on every type of request: For example, send an email notification every time a Comment is created - but not updated - whether it was created at the /comments endpoint or sideposted at the /posts endpoint.

Internal Side-Effects

For the first scenario, it’s OK to use ActiveRecord callbacks (or the equivalent functionality in a different ORM):

# app/models/user.rb
class Post < ApplicationRecord
  before_save :set_published_at,
    on: :update,
    if: :publishing?

  private

  def set_published_at
    self.published_at = Time.now
  end

  def publishing?
    status_changed? && status == 'published'
  end
end

Side-Effects on Specific Action

Just like you would with vanilla Rails, use the controller. Here we’ll only send an email to our subscribers when the Post is first created. Keep in mind we could also “sidepost” Post objects at the /blogs endpoint, but this will only fire at the /posts endpoint.

class PostsController < ApplicationController
  def create
    post, success = jsonapi_create.to_a

    if success
      PostMailer.published_email.deliver_later
      render_jsonapi(post, scope: false)
    else
      render_errors_for(post)
    end
  end
end

Side-Effects on Every Request of a Given Type

Let’s add some special logging every time we create a Post. Note this will fire every time we create a Post - whether we create it at the /posts endpoint or “sidepost” at the /blogs endpoint.

We do not need to worry about this side-effect in our model specs, or rake tasks, as the functionality is only relevant to the API.

Edit your Resource:

# app/resources/post_resource.rb
def create(attributes)
  Rails.logger.info "Post begin created by #{context.current_user.email}..."
  super
  Rails.logger.info "Success!"
end