Allow from address to fall back to parent mailer

app/mailers/passwordless/mailer.rb

@@ -3,7 +3,7 @@
 module Passwordless
   # The mailer responsible for sending Passwordless' mails.
   class Mailer < Passwordless.config.parent_mailer.constantize
-    default from: Passwordless.config.default_from_address
+    default Passwordless.config.mailer_defaults
 
     # Sends a token and a magic link
     #

lib/passwordless/config.rb

@@ -50,6 +50,12 @@ class Configuration
     def initialize
       set_defaults!
     end
+
+    def mailer_defaults
+      # Compacting the Hash removes all keys with nil values, allowing to let
+      # the `from` address fall back to the parent mailer's default.
+      { from: default_from_address }.compact
+    end
   end
 
   module Configurable

test/mailers/passwordless/mailer_test.rb

@@ -1,5 +1,9 @@
 require "test_helper"
 
+class ParentMailer < ActionMailer::Base
+  default from: "parent@mailer.com"
+end
+
 class Passwordless::MailerTest < ActionMailer::TestCase
   test("sign_in") do
     user = users(:alice)
@@ -35,4 +39,19 @@ class Passwordless::MailerTest < ActionMailer::TestCase
     assert_match /sign in: hello\n/, email.body.to_s
     assert_match %r{/admins/sign_in/#{session.identifier}/hello}, email.body.to_s
   end
+
+  test("without default_from_address falls back to parent_mailer") do
+    Passwordless.configure do |config|
+      config.default_from_address = nil
+      config.parent_mailer = "ParentMailer"
+    end
+
+    email = Passwordless::Mailer.sign_in(
+      Passwordless::Session.create!(authenticatable: users(:alice), token: "hello")
+    )
+
+    assert_equal ["parent@mailer.com"], email.from
+
+    Passwordless.reset_config!
+  end
 end