Fix fields option to restrict relationships as well.

lib/active_model/serializer/adapter/json_api.rb

@@ -174,7 +174,10 @@ module ActiveModel
         end
 
         def relationships_for(serializer)
-          serializer.associations.each_with_object({}) do |association, hash|
+          resource_type = resource_identifier_type_for(serializer)
+          requested_associations = fieldset.try(:fields_for, resource_type) || '*'
+          include_tree = IncludeTree.from_include_args(requested_associations)
+          serializer.associations(include_tree).each_with_object({}) do |association, hash|
             hash[association.key] = { data: relationship_value_for(association.serializer, association.options) }
           end
         end

test/adapter/json_api/belongs_to_test.rb

@@ -56,7 +56,7 @@ module ActiveModel
           end
 
           def test_limiting_linked_post_fields
-            @adapter = ActiveModel::Serializer::Adapter::JsonApi.new(@serializer, include: [:post], fields: { post: [:title] })
+            @adapter = ActiveModel::Serializer::Adapter::JsonApi.new(@serializer, include: [:post], fields: { post: [:title, :comments, :blog, :author] })
             expected = [{
               id: '42',
               type: 'posts',

test/adapter/json_api/collection_test.rb

@@ -60,7 +60,7 @@ module ActiveModel
           def test_limiting_fields
             actual = ActiveModel::SerializableResource.new(
               [@first_post, @second_post], adapter: :json_api,
-                                           fields: { posts: ['title'] })
+                                           fields: { posts: %w(title comments blog author) })
                      .serializable_hash
             expected = [
               {

test/adapter/json_api/fields_test.rb

@@ -0,0 +1,89 @@
+require 'test_helper'
+
+module ActiveModel
+  class Serializer
+    module Adapter
+      class JsonApi
+        class FieldsTest < Minitest::Test
+          Post = Class.new(::Model)
+          class PostSerializer < ActiveModel::Serializer
+            type 'posts'
+            attributes :title, :body
+            belongs_to :author
+            has_many :comments
+          end
+
+          Author = Class.new(::Model)
+          class AuthorSerializer < ActiveModel::Serializer
+            type 'authors'
+            attributes :name, :birthday
+          end
+
+          Comment = Class.new(::Model)
+          class CommentSerializer < ActiveModel::Serializer
+            type 'comments'
+            attributes :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 = { posts: [:title] }
+            hash = serializable(@post, adapter: :json_api, fields: fields).serializable_hash
+            expected = {
+              title: 'Title 1'
+            }
+
+            assert_equal(expected, hash[:data][:attributes])
+          end
+
+          def test_fields_relationships
+            fields = { posts: [:author] }
+            hash = serializable(@post, adapter: :json_api, fields: fields).serializable_hash
+            expected = {
+              author: {
+                data: {
+                  type: 'authors',
+                  id: '1'
+                }
+              }
+            }
+
+            assert_equal(expected, hash[:data][:relationships])
+          end
+
+          def test_fields_included
+            fields = { posts: [:author], comments: [:body] }
+            hash = serializable(@post, adapter: :json_api, fields: fields, include: 'comments').serializable_hash
+            expected = [
+              {
+                type: 'comments',
+                id: '7',
+                attributes: {
+                  body: 'cool'
+                }
+              }, {
+                type: 'comments',
+                id: '12',
+                attributes: {
+                  body: 'awesome'
+                }
+              }
+            ]
+
+            assert_equal(expected, hash[:included])
+          end
+        end
+      end
+    end
+  end
+end

test/adapter/json_api/has_many_test.rb

@@ -68,7 +68,7 @@ module ActiveModel
           end
 
           def test_limit_fields_of_linked_comments
-            @adapter = ActiveModel::Serializer::Adapter::JsonApi.new(@serializer, include: [:comments], fields: { comment: [:id] })
+            @adapter = ActiveModel::Serializer::Adapter::JsonApi.new(@serializer, include: [:comments], fields: { comment: [:id, :post, :author] })
             expected = [{
               id: '1',
               type: 'comments',