Fix deserialization of nil relationships

failing test

use try for when the assoc_data is possibly nil

rubocop test/action_controller/json_api/deserialization_test.rb -a

attempt to work on rails-master

account for rails/master having  instead of nil for assoc_data

added changelog

CHANGELOG.md

@@ -34,6 +34,7 @@ Features:
 - [#1340](https://github.com/rails-api/active_model_serializers/pull/1340) Add support for resource-level meta. (@beauby)
 
 Fixes:
+- [#1651](https://github.com/rails-api/active_model_serializers/pull/1651) Fix deserialization of nil relationships. (@NullVoxPopuli)
 - [#1480](https://github.com/rails-api/active_model_serializers/pull/1480) Fix setting of cache_store from Rails configuration. (@bf4)
   Fix uninentional mutating of value in memory cache store. (@groyoh)
 - [#1622](https://github.com/rails-api/active_model_serializers/pull/1622) Fragment cache changed from per-record to per-serializer.

lib/active_model_serializers/adapter/json_api/deserialization.rb

@@ -188,7 +188,9 @@ def parse_relationship(assoc_name, assoc_data, options)
             end
 
           polymorphic = (options[:polymorphic] || []).include?(assoc_name.to_sym)
-          hash["#{prefix_key}_type".to_sym] = assoc_data[:type] if polymorphic
+          if polymorphic
+            hash["#{prefix_key}_type".to_sym] = assoc_data.present? ? assoc_data[:type] : nil
+          end
 
           hash
         end

test/action_controller/json_api/deserialization_test.rb

@@ -9,10 +9,50 @@ def render_parsed_payload
             parsed_hash = ActiveModelSerializers::Deserialization.jsonapi_parse(params)
             render json: parsed_hash
           end
+
+          def render_polymorphic_parsed_payload
+            parsed_hash = ActiveModelSerializers::Deserialization.jsonapi_parse(
+              params,
+              polymorphic: [:restriction_for, :restricted_to]
+            )
+            render json: parsed_hash
+          end
         end
 
         tests DeserializationTestController
 
+        def test_deserialization_of_relationship_only_object
+          hash = {
+            'data' => {
+              'type' => 'restraints',
+              'relationships' => {
+                'restriction_for' => {
+                  'data' => {
+                    'type' => 'discounts',
+                    'id' => '67'
+                  }
+                },
+                'restricted_to' => {
+                  'data' => nil
+                }
+              }
+            },
+            'restraint' => {}
+          }
+
+          post :render_polymorphic_parsed_payload, params: hash
+
+          response = JSON.parse(@response.body)
+          expected = {
+            'restriction_for_id' => '67',
+            'restriction_for_type' => 'discounts',
+            'restricted_to_id' => nil,
+            'restricted_to_type' => nil
+          }
+
+          assert_equal(expected, response)
+        end
+
         def test_deserialization
           hash = {
             'data' => {