Ensure generator picks up ApplicationSerializer

ApplicationSerializer could exist, but not be loaded. So, we check
existence by looking at the filesystem instead of defined?.

Fixes rails-api#1890

CHANGELOG.md

@@ -7,6 +7,7 @@ Breaking changes:
 Fixes:
 
 - [#1887](https://github.com/rails-api/active_model_serializers/pull/1887) Make the comment reflect what the function does (@johnnymo87)
+- [#1890](https://github.com/rails-api/active_model_serializers/issues/1890) Ensure generator inherits from ApplicationSerializer when available (@richmolj)
 
 Features:
 

lib/generators/rails/serializer_generator.rb

@@ -25,7 +25,7 @@ def association_names
       def parent_class_name
         if options[:parent]
           options[:parent]
-        elsif defined?(::ApplicationSerializer)
+        elsif 'ApplicationSerializer'.safe_constantize
           'ApplicationSerializer'
         else
           'ActiveModel::Serializer'

test/generators/serializer_generator_test.rb

@@ -20,11 +20,10 @@ def test_generates_a_namespaced_serializer
   end
 
   def test_uses_application_serializer_if_one_exists
-    Object.const_set(:ApplicationSerializer, Class.new)
-    run_generator
-    assert_file 'app/serializers/account_serializer.rb', /class AccountSerializer < ApplicationSerializer/
-  ensure
-    Object.send :remove_const, :ApplicationSerializer
+    stub_safe_constantize(expected: 'ApplicationSerializer') do
+      run_generator
+      assert_file 'app/serializers/account_serializer.rb', /class AccountSerializer < ApplicationSerializer/
+    end
   end
 
   def test_uses_given_parent
@@ -54,4 +53,22 @@ def test_with_no_attributes_does_not_add_extra_space
       end
     end
   end
+
+  private
+
+  def stub_safe_constantize(expected:)
+    String.class_eval do
+      alias_method :old, :safe_constantize
+    end
+    String.send(:define_method, :safe_constantize) do
+      Class if self == expected
+    end
+
+    yield
+  ensure
+    String.class_eval do
+      alias_method :safe_constantize, :old
+      undef_method :old
+    end
+  end
 end