Add tests for fields option demonstrating usage on both attributes and relationships (#1839)

* add test for fields whitelisting relationships, and use the JSON API Include Directive to do the heavy lifting

CHANGELOG.md

@@ -15,6 +15,7 @@ Fixes:
 - [#1881](https://github.com/rails-api/active_model_serializers/pull/1881) ActiveModelSerializers::Model correctly works with string keys (@yevhene)
 
 Misc:
+- [#1839](https://github.com/rails-api/active_model_serializers/pull/1839) `fields` tests demonstrating usage for both attributes and relationships. (@NullVoxPopuli)
 
 - [#1878](https://github.com/rails-api/active_model_serializers/pull/1878) Cache key generation for serializers now uses `ActiveSupport::Cache.expand_cache_key` instead of `Array#join` by default and is also overridable. This change should be backward-compatible. (@markiz)
 

test/action_controller/json_api/fields_test.rb

@@ -0,0 +1,57 @@
+require 'test_helper'
+
+module ActionController
+  module Serialization
+    class JsonApi
+      class FieldsTest < ActionController::TestCase
+        class FieldsTestController < ActionController::Base
+          class PostSerializer < ActiveModel::Serializer
+            type 'posts'
+            attributes :title, :body, :publish_at
+            belongs_to :author
+            has_many :comments
+          end
+
+          def setup_post
+            ActionController::Base.cache_store.clear
+            @author = Author.new(id: 1, first_name: 'Bob', last_name: 'Jones')
+            @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],
+                             publish_at: '2020-03-16T03:55:25.291Z')
+            @comment1.post = @post
+            @comment2.post = @post
+          end
+
+          def render_fields_works_on_relationships
+            setup_post
+            render json: @post, serializer: PostSerializer, adapter: :json_api, fields: { posts: [:author] }
+          end
+        end
+
+        tests FieldsTestController
+
+        test 'fields works on relationships' do
+          get :render_fields_works_on_relationships
+          response = JSON.parse(@response.body)
+          expected = {
+            'data' => {
+              'id' => '1337',
+              'type' => 'posts',
+              'relationships' => {
+                'author' => {
+                  'data' => {
+                    'id' => '1',
+                    'type' => 'authors'
+                  }
+                }
+              }
+            }
+          }
+          assert_equal expected, response
+        end
+      end
+    end
+  end
+end

test/adapter/json_api/has_many_test.rb

@@ -40,6 +40,27 @@ module ActiveModelSerializers
           assert_equal(expected, @adapter.serializable_hash[:data][:relationships][:comments])
         end
 
+        test 'relationships can be whitelisted via fields' do
+          @adapter = ActiveModelSerializers::Adapter::JsonApi.new(@serializer, fields: { posts: [:author] })
+          result = @adapter.serializable_hash
+          expected = {
+            data: {
+              id: '1',
+              type: 'posts',
+              relationships: {
+                author: {
+                  data: {
+                    id: '1',
+                    type: 'authors'
+                  }
+                }
+              }
+            }
+          }
+
+          assert_equal expected, result
+        end
+
         def test_includes_linked_comments
           @adapter = ActiveModelSerializers::Adapter::JsonApi.new(@serializer, include: [:comments])
           expected = [{