How to test instance variables in an ActionMailer object?

Question

There is a before_action callback method in my ActionMailer object which is responsible for setting some instance variables.

class TestMailer < ApplicationMailer
   before_action :set_params

   def send_test_mail
     mail(to: @email, subject: subject)
   end

   def set_params
     @account = account.email 
     @date = some_action(account.updated_at)
   end
end

The question is How one can test these variables in a rspec test? some thing like:

describe TestMailer do 
 describe '#set_params' do
   described_class.with(account: account, subject: subject).send_test_mail.deliver_now
   expect(@date).to eq(Date.today)
 end
end

any clue would be highly appreciated.

Answer 1

you could setup a spy inside a mock method instance_variable_set, then validate that spy

class TestMailer < ApplicationMailer
 attr_accessor :day
 # ...
end

describe TestMailer do 
 let(:freeze_today) { Time.now.utc }

 it '#set_params' do 
  # freeze today
  allow_any_instance_of(TestMailer).to receive(:some_action)
  .with(account.updated_at)
  .and_return(freeze_today)

  # spy
  @spy = nil
  allow_any_instance_of(TestMailer).to receive(:day=) do |time|
   @spy = time
  end

  described_class.with(account: account, subject: subject)
  .send_test_mail
  .deliver_now

  expect(@spy).to eq(freeze_today)

  # or just simple like this  
  expect_any_instance_of(TestMailer).to receive(:day=).with(freeze_today)
 end
end