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

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