show in a user profile who the user was invited by

app/models/user.rb

@@ -9,6 +9,8 @@ class User < ActiveRecord::Base
     :class_name => "Message",
     :foreign_key => "recipient_user_id"
   has_many :tag_filters
+  belongs_to :invited_by_user,
+    :class_name => "User"
 
   has_secure_password
 

app/views/users/show.html.erb

@@ -21,7 +21,14 @@
 
   <label class="required">Joined:</label>
   <span class="d">
-    <%= time_ago_in_words(@showing_user.created_at) %> ago
+    <%= time_ago_in_words(@showing_user.created_at) %>
+    <% if @showing_user.invited_by_user %>
+      ago, invited by
+      <a href="/u/<%= @showing_user.invited_by_user.username %>"><%=
+        @showing_user.invited_by_user.username %></a>
+    <% else %>
+      ago
+    <% end %>
   </span>
   <br>