invitations_controller_test.rb

      the 'create action' do                                                     
        def post_create(params = {})                                             
          post :create, invitation: params                                       
        end                                                                      
                                                                                 
        def valid_params                                                         
          { email: 'user@domain.com', first_name: 'First', last_name: 'Last' }   
        end  
        
        it 'should send an email to invite new users' do                                              
          assert_difference 'ActionMailer::Base.deliveries.size' do                 
            post_create valid_params                                                
          end                                                                       
        end   
        
        # lots of other tests removed for clarity
      end

signup_mailer_test.rb

  an 'Account invitation email' do                                                                    
    before do                                                                       
      @code = Invitation.generate_code                                              
      invitation = build(                                                           
          :invitation, kind: 'account', email: @email, code: @code)                 
      @message = SignupMailer.account_invite(invitation).deliver                    
    end                                                                             
                                                                                    
    it 'should be sent to the invitee' do                                           
      assert_equal [@email], @message.to                                            
      assert_equal 'Invite code for The Agile Planner', @message.subject            
    end                                                                             
                                                                                    
    it 'should contain a link to the signup page' do                                
      assert_match %r{com/signup\?code=#{@code}}, @message.encoded                  
    end                                                                             
  end