Using MiniTest to mock and test ActiveRecord callbacks in Rails 4.2

team_membership.rb

class TeamMembership < ActiveRecord::Base
  # A proc that will enqueue `NotificationMailer.team_invitation`
  DEFAULT_NOTIFIER = proc do |user, team|
    NotificationMailer.team_invitation(team, user).deliver_later
  end

  class << self
    # This is a class level attribute that is mainly used for testing.
    # Defaults to {TeamMembership::DEFAULT_NOTIFIER}
    attr_accessor :notifier
  end
  self.notifier = DEFAULT_NOTIFIER

  belongs_to :team
  belongs_to :user

  after_create :invite_user_to_team!

  private

  # ActiveRecord callback used to equeue team invitation emails
  # @return void
  def invite_user_to_team!
    self.class.notifier.call(team, user)
    nil
  end
end

team_membership_test.rb

require 'test_helper'

class TeamMemberTest < ActiveSupport::TestCase
  test 'callbacks' do
    # setup
    test_notifier = Minitest::Mock.new
    test_notifier.expect(:call, nil, [teams(:alpha), users(:carol)])
    TeamMembership.notifier = test_notifier

    # test
    TeamMembership.create(user_id: users(:carol).id, team_id: teams(:alpha).id)
    assert test_notifier.verify

    # teardown
    TeamMembership.notifier = TeamMembership::DEFAULT_NOTIFIER
  end
end

@benkoshy This was written for Rails cira version 4.2 A lot has changed since then, so I would not be surprised if there are better ways of testing/asserting desired behavior for stuff like this.

Looking at Rails Guides: ActiveRecord Callbacks: 9. Callback Classes, if I were to do this today I'd write a tests around an AR callback class and test that independently of the AR model it was used in. Depending on your needs, your AR callback class could take arguents on it's initialization method if you needed to make it configurable or wanted to pass in mocks during testing:

# AR Model
class TeamMembership < ActiveRecord::Base
  belongs_to :team
  belongs_to :user

  after_create EmailNotificationCallback.new( NotificationMailer.method(:team_invitation) )
end

# AR Callback
class EmailNotificationCallback
  def initialize(mailer_proc, **mailer_args)
    @mailer = mailer_proc
    @mailer_args = mailer_args
  end

  def after_create(membership)
    mailer.call(membership.user, membership.team)
  end
end

# AR Model Callback Test
# All we need to do is verify that our callback class is called
class TeamMembershipTest < ActiveSupport::TestCase
  test 'callbacks' do
    # setup
    test_callback = Minitest::Mock.new
    test_callback.expect(:new, nil, [teams(:alpha), users(:carol)])

    # test
    EmailNotificationCallback.stub do
      TeamMembership.create(user_id: users(:carol).id, team_id: teams(:alpha).id)
    end

    assert test_callback.verify # Assert that our AR Callback was executed
  end
end

# AR Callback Test
# We can now test the callback in isolation since we've verified the model <-> callback hook up in a previous test
class EmailNotificationCallback < ActiveSupport::TestCase
  test 'after_create' do
    # setup
    mailer_proc = Minitest::Mock.new
    mailer_proc.expect(:call, nil, [teams(:alpha), users(:carol)])

    membership = TeamMembership.create(user_id: users(:carol).id, team_id: teams(:alpha).id)

    instance = EmailNotificationCallback.new(mailer_proc)

    # test
    instance.after_create(membership)
    mailer_proc.verify # assert our mailer gets called with expected arguments
  end
end

Note: Code is untested, but should demonstrate my thoughts well enough.

@damien, i appreciate the suggestions. I will implement as you've suggested. Thank you!