Prevent creation of unnecessary fieldset(mirror and revise changes from

rails-api#2370)

Co-authored-by: https://github.com/abcang

lib/active_model_serializers/adapter/attributes.rb

@@ -5,7 +5,11 @@ module Adapter
     class Attributes < Base
       def initialize(*)
         super
-        instance_options[:fieldset] ||= ActiveModel::Serializer::Fieldset.new(fields_to_fieldset(instance_options.delete(:fields)))
+
+        instance_options[:fieldset] ||= begin
+          fieldset = fields_to_fieldset(instance_options.delete(:fields))
+          fieldset && ActiveModel::Serializer::Fieldset.new(fieldset)
+        end
       end
 
       def serializable_hash(options = nil)

lib/active_model_serializers/adapter/json_api.rb

@@ -53,7 +53,7 @@ def self.fragment_cache(cached_hash, non_cached_hash, root = true)
       def initialize(serializer, options = {})
         super
         @include_directive = JSONAPI::IncludeDirective.new(options[:include], allow_wildcard: true)
-        @fieldset = options[:fieldset] || ActiveModel::Serializer::Fieldset.new(options.delete(:fields))
+        @fieldset = options[:fieldset] || ((option_fields = options.delete(:fields)) && ActiveModel::Serializer::Fieldset.new(option_fields))
       end
 
       # {http://jsonapi.org/format/#crud Requests are transactional, i.e. success or failure}
@@ -348,7 +348,8 @@ def data_for(serializer, include_slice)
         data.tap do |resource_object|
           next if resource_object.nil?
           # NOTE(BF): the attributes are cached above, separately from the relationships, below.
-          requested_associations = fieldset.fields_for(resource_object[:type]) || '*'
+          requested_fields = fieldset && fieldset.fields_for(resource_object[:type])
+          requested_associations = requested_fields || '*'
           relationships = relationships_for(serializer, requested_associations, include_slice)
           resource_object[:relationships] = relationships if relationships.any?
         end

test/serializers/attribute_test.rb

@@ -27,6 +27,20 @@ def test_attribute_inheritance_with_key
         assert_equal({ id: 1, title: 'AMS Hints' }, adapter.serializable_hash)
       end
 
+      def test_attribute_initialization_with_empty_fields
+        inherited_klass = Class.new(AlternateBlogSerializer)
+        blog_serializer = inherited_klass.new(@blog)
+        adapter = ActiveModelSerializers::Adapter::Attributes.new(blog_serializer, {fieldset: nil, fields: nil})
+        assert_nil(adapter.instance_options[:fieldset])
+      end
+
+      def test_attribute_initialization_with_present_fields
+        inherited_klass = Class.new(AlternateBlogSerializer)
+        blog_serializer = inherited_klass.new(@blog)
+        adapter = ActiveModelSerializers::Adapter::Attributes.new(blog_serializer, {fieldset: nil, fields: [:id]})
+        assert_equal(ActiveModel::Serializer::Fieldset, adapter.instance_options[:fieldset].class)
+      end
+
       def test_multiple_calls_with_the_same_attribute
         serializer_class = Class.new(ActiveModel::Serializer) do
           attribute :title