Use count rather than size to count posts efficiently

app/views/feeds/index.html.haml

@@ -17,7 +17,7 @@
       %td
         .feed_title
           = link_to f.title, f
-      %td.right= f.posts.size
+      %td.right= f.posts.count
       %td
         - if f.last_fetched.nil?
           never

app/views/feeds/show.html.haml

@@ -9,7 +9,7 @@
     Validate
 
 %p
-  = pluralize(@feed.posts.size, "post")
+  = pluralize(@feed.posts.count, "post")
 
 %p
   = link_to @feed.feed_url, @feed.feed_url