How to test Rails mailers using RSpec

class Notifier < ActionMailer::Base
  default from: '[email protected]'

  def instructions(user)
    @name = user.name
    @confirmation_url = confirmation_url(user)

    mail to: user.email, subject: 'Instructions'
  end
end

class User
  def send_instructions
    Notifier.instructions(self).deliver_now
  end
end

Rails.application.configure do
  config.action_mailer.delivery_method = :test
end

# spec/models/user_spec.rb

RSpec.describe User, type: :model do
  subject { create :user }

  it 'sends an email' do
    expect { subject.send_instructions }
      .to change { ActionMailer::Base.deliveries.count }.by(1)
  end
end

# spec/mailers/notifier_spec.rb

RSpec.describe Notifier, type: :mailer do
  describe 'instructions' do
    let(:user) { mock_model User, name: 'Lucas', email: '[email protected]' }
    let(:mail) { described_class.instructions(user).deliver_now }

    it 'renders the subject' do
      expect(mail.subject).to eq('Instructions')
    end

    it 'renders the receiver email' do
      expect(mail.to).to eq([user.email])
    end

    it 'renders the sender email' do
      expect(mail.from).to eq(['[email protected]'])
    end

    it 'assigns @name' do
      expect(mail.body.encoded).to match(user.name)
    end

    it 'assigns @confirmation_url' do
      expect(mail.body.encoded)
        .to match("http://aplication_url/#{user.id}/confirmation")
    end
  end
end