Apply records_for_RELATIONSHIP method on polymorphic to many linkage

Currently we can use `records_for_RELATIONSHIP` to control the included
records. But that change doesn't apply to the linkage data. So we'll get
a consistent result between included records and linkage data. This
commit fixes the issue.

Closes gh-1239

lib/jsonapi/resource_serializer.rb

@@ -394,12 +394,16 @@ def to_one_linkage(source, relationship)
 
     def to_many_linkage(source, relationship)
       linkage = []
+      include_config = include_directives.include_config(relationship.name.to_sym) if include_directives
+      include_filters = include_config[:include_filters] if include_config
+      options = { filters: include_filters || {} }
+
       linkage_types_and_values = if source.preloaded_fragments.has_key?(format_key(relationship.name))
         source.preloaded_fragments[format_key(relationship.name)].map do |_, resource|
           [relationship.type, resource.id]
         end
       elsif relationship.polymorphic?
-        assoc = source._model.public_send(relationship.name)
+        assoc = source.public_send("records_for_#{relationship.name}", options)
         # Avoid hitting the database again for values already pre-loaded
         if assoc.respond_to?(:loaded?) and assoc.loaded?
           assoc.map do |obj|
@@ -411,9 +415,6 @@ def to_many_linkage(source, relationship)
           end
         end
       else
-        include_config = include_directives.include_config(relationship.name.to_sym) if include_directives
-        include_filters = include_config[:include_filters] if include_config
-        options = { filters: include_filters || {} }
         source.public_send(relationship.name, options).map do |value|
           [relationship.type, value.id]
         end

test/unit/serializer/polymorphic_serializer_test.rb

@@ -130,6 +130,73 @@ def test_sti_polymorphic_to_many_serialization
     )
   end
 
+  def test_sti_polymorphic_to_many_serialization_with_custom_polymorphic_records
+    person_resource = PersonResource.new(@person, nil)
+    serializer = JSONAPI::ResourceSerializer.new(
+      PersonResource,
+      include: %w(vehicles)
+    )
+
+    def person_resource.records_for_vehicles(opts = {})
+      @model.vehicles.none
+    end
+
+    serialized_data = serializer.serialize_to_hash(person_resource)
+
+    assert_hash_equals(
+      {
+        data: {
+          id: '1',
+          type: 'people',
+          links: {
+            self: '/people/1'
+          },
+          attributes: {
+            name: 'Joe Author',
+            email: '[email protected]',
+            dateJoined: '2013-08-07 16:25:00 -0400'
+          },
+          relationships: {
+            comments: {
+              links: {
+                self: '/people/1/relationships/comments',
+                related: '/people/1/comments'
+              }
+            },
+            posts: {
+              links: {
+                self: '/people/1/relationships/posts',
+                related: '/people/1/posts'
+              }
+            },
+            vehicles: {
+              links: {
+                self: '/people/1/relationships/vehicles',
+                related: '/people/1/vehicles'
+              },
+              :data => []
+            },
+            preferences: {
+              links: {
+                self: '/people/1/relationships/preferences',
+                related: '/people/1/preferences'
+              }
+            },
+            hairCut: {
+              links: {
+                self: '/people/1/relationships/hairCut',
+                related: '/people/1/hairCut'
+              }
+            }
+          }
+        }
+      },
+      serialized_data
+    )
+  end
+
+
+
   def test_polymorphic_belongs_to_serialization
     serialized_data = JSONAPI::ResourceSerializer.new(
       PictureResource,