Move original_url from translation to problem

app/models/problem.rb

@@ -10,4 +10,8 @@ class Problem < ActiveRecord::Base
     !!self.translation
   end
 
+
+  def original_url
+    "https://projecteuler.net/problem=#{self.id}"
+  end
 end

app/models/translation.rb

@@ -5,8 +5,4 @@ class Translation < ActiveRecord::Base
   validates :title, uniqueness: true
 
   self.per_page = 50
-
-  def original_url
-    "https://projecteuler.net/problem=#{id}"
-  end
 end

app/views/problems/show.html.erb

@@ -12,5 +12,5 @@
     </div>
 <% end %>
 <div class="text-center">
-  <%= link_to 'Dieses Problem auf projecteuler.net', @problem.translation.original_url, target: '_blank' %>
+  <%= link_to 'Dieses Problem auf projecteuler.net', @problem.original_url, target: '_blank' %>
 </div>

app/views/translations/show.html.erb

@@ -3,7 +3,7 @@
 <% end %>
 
 <div class="page-header">
-  <h1><%= @translation.title %> <small>Problem <%= @translation.id %></small></h1>
+  <h1><%= @translation.title %> <small>Problem <%= @translation.problem_id %></small></h1>
 </div>
 
 <%= panel do %>
@@ -12,6 +12,6 @@
     </div>
 <% end %>
 <div class="text-center">
-  <%= link_to 'Dieses Problem auf projecteuler.net', @translation.original_url, target: '_blank' %>
+  <%= link_to 'Dieses Problem auf projecteuler.net', @translation.problem.original_url, target: '_blank' %>
 </div>
 <%= link_to 'Edit', edit_translation_path(@translation), class: 'btn btn-default' %>