Add migration to add custom_url field to BlogPost.

db/migrate/5_add_custom_url_field_to_blog_posts.rb

@@ -0,0 +1,9 @@
+class AddCustomUrlFieldToBlogPosts < ActiveRecord::Migration
+  def self.up
+    add_column :blog_posts, :custom_url, :string
+  end
+
+  def self.down
+    remove_column :blog_posts, :custom_url
+  end
+end