Follows https://github.com/rails-api/active_model_serializers/pull/2100

lib/active_model/serializer.rb

@@ -365,6 +365,9 @@ module ActiveModel
     def serializable_hash(adapter_options = nil, options = {}, adapter_instance = self.class.serialization_adapter_instance)
       adapter_options ||= {}
       options[:include_directive] ||= ActiveModel::Serializer.include_directive_from_options(adapter_options)
+      if (fieldset = adapter_options[:fieldset])
+        options[:fields] = fieldset.fields_for(json_key)
+      end
       resource = attributes_hash(adapter_options, options, adapter_instance)
       relationships = associations_hash(adapter_options, options, adapter_instance)
       resource.merge(relationships)
@@ -379,7 +382,12 @@ module ActiveModel
 
     # Used by adapter as resource root.
     def json_key
-      root || _type || object.class.model_name.to_s.underscore
+      root || _type ||
+        begin
+          object.class.model_name.to_s.underscore
+        rescue ArgumentError
+          'anonymous_object'
+        end
     end
 
     def read_attribute_for_serialization(attr)

lib/active_model_serializers/adapter/attributes.rb

@@ -3,6 +3,11 @@
 module ActiveModelSerializers
   module Adapter
     class Attributes < Base
+      def initialize(*)
+        super
+        instance_options[:fieldset] ||= ActiveModel::Serializer::Fieldset.new(fields_to_fieldset(instance_options.delete(:fields)))
+      end
+
       def serializable_hash(options = nil)
         options = serialization_options(options)
         options[:fields] ||= instance_options[:fields]
@@ -10,6 +15,20 @@ module ActiveModelSerializers
 
         self.class.transform_key_casing!(serialized_hash, instance_options)
       end
+
+      def fields_to_fieldset(fields)
+        return fields if fields.nil?
+        resource_fields = []
+        relationship_fields = {}
+        fields.each do |field|
+          case field
+          when Symbol, String then resource_fields << field
+          when Hash then relationship_fields.merge!(field)
+          else fail ArgumentError, "Unknown conversion of fields to fieldset: '#{field.inspect}' in '#{fields.inspect}'"
+          end
+        end
+        relationship_fields.merge(serializer.json_key.to_sym => resource_fields)
+      end
     end
   end
 end

test/adapter/json/fields_test.rb

@@ -0,0 +1,63 @@
+require 'test_helper'
+
+module ActiveModelSerializers
+  module Adapter
+    class Json
+      class FieldsTest < ActiveSupport::TestCase
+        class Post < ::Model
+          attributes :title, :body
+          associations :author, :comments
+        end
+        class Author < ::Model
+          attributes :name, :birthday
+        end
+        class Comment < ::Model
+          attributes :title, :body
+          associations :author, :post
+        end
+
+        class PostSerializer < ActiveModel::Serializer
+          type 'posts'
+          attributes :title, :body
+          belongs_to :author
+          has_many :comments
+        end
+
+        class AuthorSerializer < ActiveModel::Serializer
+          attributes :name, :birthday
+        end
+
+        class CommentSerializer < ActiveModel::Serializer
+          type 'comments'
+          attributes :title, :body
+          belongs_to :author
+        end
+
+        def setup
+          @author = Author.new(id: 1, name: 'Lucas', birthday: '10.01.1990')
+          @comment1 = Comment.new(id: 7, body: 'cool', author: @author)
+          @comment2 = Comment.new(id: 12, body: 'awesome', author: @author)
+          @post = Post.new(id: 1337, title: 'Title 1', body: 'Body 1',
+                           author: @author, comments: [@comment1, @comment2])
+          @comment1.post = @post
+          @comment2.post = @post
+        end
+
+        def test_fields_attributes
+          fields = [:title]
+          hash = serializable(@post, adapter: :json, fields: fields, include: []).serializable_hash
+          expected = { title: 'Title 1' }
+          assert_equal(expected, hash[:posts])
+        end
+
+        def test_fields_included
+          fields = [:title, { comments: [:body] }]
+          hash = serializable(@post, adapter: :json, include: [:comments], fields: fields).serializable_hash
+          expected = [{ body: @comment1.body }, { body: @comment2.body }]
+
+          assert_equal(expected, hash[:posts][:comments])
+        end
+      end
+    end
+  end
+end