fix(Relationship): do not set static_record_type for polymorphic relationships (#82)

CHANGELOG.md

@@ -4,6 +4,10 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [unreleased]
+### Fixed
+- Relationship#record_type_for does not assign static record type for polymorphic relationships (#83)
+
 ## [1.7.1] - 2020-05-01
 ### Fixed
 - ObjectSerializer#serialized_json accepts arguments for to_json (#80)

lib/fast_jsonapi/relationship.rb

@@ -212,8 +212,13 @@ module FastJsonapi
     end
 
     def compute_static_record_type
-      return run_key_transform(record_type) if record_type
-      return run_key_transform(@static_serializer.record_type) if @static_serializer
+      if polymorphic
+        nil
+      elsif record_type
+        run_key_transform(record_type)
+      elsif @static_serializer
+        run_key_transform(@static_serializer.record_type)
+      end
     end
   end
 end