Run the bug

lib/jsonapi/relationship.rb

@@ -59,18 +59,40 @@ def table_name
 
     def self.polymorphic_types(name)
       @poly_hash ||= {}.tap do |hash|
-        ObjectSpace.each_object do |klass|
-          next unless Module === klass
-          if ActiveRecord::Base > klass
-            klass.reflect_on_all_associations(:has_many).select{|r| r.options[:as] }.each do |reflection|
-              (hash[reflection.options[:as]] ||= []) << klass.name.downcase
-            end
+        candidate_polymorphic_classes.each do |klass|
+          klass.reflect_on_all_associations(:has_many).select{|r| r.options[:as] }.each do |reflection|
+            (hash[reflection.options[:as]] ||= []) << klass.name.downcase
           end
         end
       end
       @poly_hash[name.to_sym]
     end
 
+    def self.candidate_polymorphic_classes
+      candidate_polymorphic_classes = []
+      ObjectSpace.each_object do |klass|
+        next unless Module === klass
+        if ActiveRecord::Base > klass
+          if !klass.name.nil?
+            candidate_polymorphic_classes << klass
+          else
+            model_name =
+              if klass.respond_to?(:model_name)
+                begin
+                  klass.model_name.name
+                rescue ArgumentError => e
+                  "Responds to ActiveModel::Naming but #{e.message}"
+                end
+              else
+                "Does not extend ActiveModel::Naming"
+              end
+            warn "No class name found for #{klass} (#{model_name})"
+          end
+        end
+      end
+      candidate_polymorphic_classes
+    end
+
     def resource_types
       if polymorphic? && belongs_to?
         @polymorphic_types ||= self.class.polymorphic_types(@relation_name).collect {|t| t.pluralize}

test/bug-1305.rb

@@ -0,0 +1,42 @@
+require File.expand_path('../test_helper', __FILE__)
+
+# Replace this with the code necessary to make your test fail.
+class BugTest < Minitest::Test
+  include Rack::Test::Methods
+
+  def json_api_headers
+    {'Accept' => JSONAPI::MEDIA_TYPE, 'CONTENT_TYPE' => JSONAPI::MEDIA_TYPE}
+  end
+
+  def teardown
+    Individual.delete_all
+    ContactMedium.delete_all
+  end
+
+  def test_find_party_via_contact_medium
+    individual = Individual.create(name: 'test')
+    contact_medium = ContactMedium.create(party: individual, name: 'test contact medium')
+    fetched_party = contact_medium.party
+    assert_same individual, fetched_party, "Expect an individual to have been found via contact medium model's relationship 'party'"
+  end
+
+  def test_get_individual
+    individual = Individual.create(name: 'test')
+    ContactMedium.create(party: individual, name: 'test contact medium')
+    get "/individuals/#{individual.id}"
+    assert last_response.ok?
+  end
+
+  def test_get_party_via_contact_medium
+    individual = Individual.create(name: 'test')
+    contact_medium = ContactMedium.create(party: individual, name: 'test contact medium')
+    get "/contact_media/#{contact_medium.id}/party"
+    assert last_response.ok?, "Expect an individual to have been found via contact medium resource's relationship 'party'"
+  end
+
+  private
+
+  def app
+    Rails.application
+  end
+end

test/fixtures/active_record.rb

@@ -429,6 +429,19 @@
     t.integer :version
     t.timestamps null: false
   end
+
+  create_table :contact_media do |t|
+    t.string :name
+    t.references :party, polymorphic: true, index: true
+  end
+
+  create_table :individuals do |t|
+    t.string :name
+  end
+
+  create_table :organizations do |t|
+    t.string :name
+  end
 end
 
 ### MODELS
@@ -643,6 +656,22 @@ class Fact < ActiveRecord::Base
 class Like < ActiveRecord::Base
 end
 
+ class TestApplicationRecord < ActiveRecord::Base
+  self.abstract_class = true
+end
+
+class ContactMedium < TestApplicationRecord
+  belongs_to :party, polymorphic: true, inverse_of: :contact_media
+end
+
+class Individual < TestApplicationRecord
+  has_many :contact_media, as: :party
+end
+
+class Organization < TestApplicationRecord
+  has_many :contact_media, as: :party
+end
+
 class Breed
   include ActiveModel::Model
 
@@ -1246,6 +1275,18 @@ class IndicatorsController < JSONAPI::ResourceController
 class RobotsController < JSONAPI::ResourceController
 end
 
+class IndividualsController < BaseController
+end
+
+class OrganizationsController < BaseController
+end
+
+class ContactMediaController < BaseController
+end
+
+class PartiesController < BaseController
+end
+
 ### RESOURCES
 class BaseResource < JSONAPI::Resource
   abstract
@@ -2688,6 +2729,24 @@ class RobotResource < ::JSONAPI::Resource
   end
 end
 
+class ContactMediumResource < JSONAPI::Resource
+  attribute :name
+  has_one :party, polymorphic: true
+end
+
+class IndividualResource < JSONAPI::Resource
+  attribute :name
+  has_many :contact_media
+end
+
+class OrganizationResource < JSONAPI::Resource
+  attribute :name
+  has_many :contact_media
+end
+
+class PartyResource < JSONAPI::Resource
+end
+
 ### PORO Data - don't do this in a production app
 $breed_data = BreedData.new
 $breed_data.add(Breed.new(0, 'persian'))

test/test_helper.rb

@@ -450,6 +450,16 @@ class CatResource < JSONAPI::Resource
 
   mount MyEngine::Engine => "/boomshaka", as: :my_engine
   mount ApiV2Engine::Engine => "/api_v2", as: :api_v2_engine
+
+  jsonapi_resources :contact_media do
+    jsonapi_relationships
+  end
+  jsonapi_resources :individuals do
+    jsonapi_relationships
+  end
+  jsonapi_resources :organizations do
+    jsonapi_relationships
+  end
 end
 
 MyEngine::Engine.routes.draw do