Fix nested include attributes

When the requests asked for a nested attribute in the `include` and it's
missing or empty, don't break because the type of the object can't be
determined.

If the request is for a collection and none of the elements has the
attribute, it will not be added to the `linked` key, similar to what
happens with simple includes.

lib/active_model/serializer/adapter/json_api.rb

@@ -68,8 +68,8 @@ module ActiveModel
 
           resource_path = [parent, resource_name].compact.join('.')
 
-          if include_assoc?(resource_path)
-            plural_name = serialized_object_type(serializers).pluralize.to_sym
+          if include_assoc?(resource_path) && resource_type = serialized_object_type(serializers)
+            plural_name = resource_type.pluralize.to_sym
             @top[:linked] ||= {}
             @top[:linked][plural_name] ||= []
 

test/action_controller/json_api_linked_test.rb

@@ -26,6 +26,9 @@ module ActionController
           @first_comment.author = @author2
           @second_comment.post = @post
           @second_comment.author = nil
+          @post2 = Post.new(id: 2, title: "Another Post", body: "Body")
+          @post2.author = @author
+          @post2.comments = []
         end
 
         def render_resource_without_include
@@ -48,6 +51,18 @@ module ActionController
           render json: @post, include: 'author,author.roles', adapter: :json_api
         end
 
+        def render_resource_with_missing_nested_has_many_include
+          setup_post
+          @post.author = @author2 # author2 has no roles.
+          render json: @post, include: 'author,author.roles', adapter: :json_api
+        end
+
+        def render_collection_with_missing_nested_has_many_include
+          setup_post
+          @post.author = @author2
+          render json: [@post, @post2], include: 'author,author.roles', adapter: :json_api
+        end
+
         def render_collection_without_include
           setup_post
           render json: [@post], adapter: :json_api
@@ -124,6 +139,20 @@ module ActionController
         response = JSON.parse(@response.body)
         assert response.key? 'linked'
       end
+
+      def test_render_resource_with_nested_attributes_even_when_missing_associations
+        get :render_resource_with_missing_nested_has_many_include
+        response = JSON.parse(@response.body)
+        assert response.key? 'linked'
+        refute response['linked'].key? 'roles'
+      end
+
+      def test_render_collection_with_missing_nested_has_many_include
+        get :render_collection_with_missing_nested_has_many_include
+        response = JSON.parse(@response.body)
+        assert response.key? 'linked'
+        assert response['linked'].key? 'roles'
+      end
     end
   end
 end