Use find instead of find_by_id

This method will raise an exception if resource is not found when
trying to call score_action on nil.
Prefer to raise a 404 HTML NotFound error instead.

app/controllers/related_contents_controller.rb

@@ -31,7 +31,7 @@ class RelatedContentsController < ApplicationController
   private
 
   def score(action)
-    @related = RelatedContent.find_by(id: params[:id])
+    @related = RelatedContent.find params[:id]
     @related.send("score_#{action}", current_user)
 
     render template: 'relationable/_refresh_score_actions'

spec/controllers/related_contents_controller_spec.rb

@@ -0,0 +1,15 @@
+require "rails_helper"
+
+describe RelatedContentsController do
+
+  describe "#score" do
+    it "raises an error if related content does not exist" do
+      controller.params[:id] = 0
+
+      expect do
+        controller.send(:score, "action")
+      end.to raise_error ActiveRecord::RecordNotFound
+    end
+  end
+
+end