Merge pull request #2337 from InteNs/fix-json-api-belongs-to-fk-on-object

fix incorrect belongs_to serialization when foreign_key on object and belongs_to is blank

CHANGELOG.md

@@ -15,6 +15,8 @@ Fixes:
     - Fix Rails 6.0 deprication warnings
     - update test fixture schema to use `timestamps` instead of `timestamp`
 - [#2223](https://github.com/rails-api/active_model_serializers/pull/2223) Support Fieldset in Attributes/JSON adapters documented in [docs/general/fields.md](https://github.com/rails-api/active_model_serializers/blob/0-10-stable/docs/general/fields.md) that worked partially before (@bf4)
+- [#2337](https://github.com/rails-api/active_model_serializers/pull/2337) fix incorrect belongs_to serialization when foreign_key on object and belongs_to is blank (@InteNs)
+    - Fixes incorrect json-api generation when `jsonapi_use_foreign_key_on_belongs_to_relationship` is `true` and the relationship is blank
 
 Misc:
 

lib/active_model_serializers/adapter/json_api/resource_identifier.rb

@@ -13,7 +13,7 @@ module ActiveModelSerializers
           type = inflect_type(type)
           type = type_for(:no_class_needed, type, options)
           if id.blank?
-            { type: type }
+            nil
           else
             { id: id.to_s, type: type }
           end

test/action_controller/json_api/linked_test.rb

@@ -82,7 +82,7 @@ module ActionController
 
           def render_collection_with_include
             setup_post
-            render json: [@post], adapter: :json_api, include: 'author, comments'
+            render json: [@post], adapter: :json_api, include: 'author,comments'
           end
         end
 

test/adapter/json_api/relationship_test.rb

@@ -35,6 +35,24 @@ module ActiveModelSerializers
           assert_equal(expected, actual)
         end
 
+        def test_relationship_with_nil_model_and_belongs_to_id_on_self
+          original_config = ActiveModelSerializers.config.jsonapi_use_foreign_key_on_belongs_to_relationship
+          ActiveModelSerializers.config.jsonapi_use_foreign_key_on_belongs_to_relationship = true
+
+          expected = { data: nil }
+
+          model_attributes = { blog: nil }
+          relationship_name = :blog
+          model = new_model(model_attributes)
+          actual = build_serializer_and_serialize_relationship(model, relationship_name) do
+            belongs_to :blog
+          end
+
+          assert_equal(expected, actual)
+        ensure
+          ActiveModelSerializers.config.jsonapi_use_foreign_key_on_belongs_to_relationship = original_config
+        end
+
         def test_relationship_with_data_array
           expected = {
             data: [

test/adapter/json_api/type_test.rb

@@ -147,8 +147,7 @@ module ActiveModelSerializers
         def test_for_type_with_id_given_blank_id
           id = ''
           actual = ResourceIdentifier.for_type_with_id('admin_user', id, {})
-          expected = { type: 'admin-users' }
-          assert_equal actual, expected
+          assert_nil actual
         end
 
         def test_for_type_with_id_inflected