be_observed_by matcher

be_observed_by.rb

require 'rspec/expectations'

RSpec::Matchers.define :be_observed_by do |observer_name|
  match do |obj|
    ActiveRecord::Base.observers.enable observer_name

    observer = observer_class(observer_name)

    # save old after_create and define new
    was_defined = observer.method_defined?(:after_create)

    unless was_defined
      observer.class_eval do
        define_method(:after_create) {}
      end

      observer.instance.instance_eval do
        define_callbacks(obj.class)
      end
    end

    observer.instance.should_receive(:after_create)

    obj.save(validate: false)

    # revert monkey patching
    unless was_defined
      observer.class_eval do
        remove_method(:after_create)
      end

      observer.instance.instance_eval do
        define_callbacks(obj.class)
      end
    end

    begin
      RSpec::Mocks::verify  # run mock verifications
      true
    rescue RSpec::Mocks::MockExpectationError => e
      # here one can use #{e} to construct an error message
      false
    end
  end

  failure_message_for_should do
    "expected #{actual.class} to be observed by #{observer_class(observer_name)}"
  end

  def observer_class(observer_name)
    observer_name.to_s.camelize.constantize
  end
end