Add support for commenting on, and replying to, diary entries.

app/models/diary_comment.rb

@@ -0,0 +1,7 @@
+class DiaryComment < ActiveRecord::Base
+  belongs_to :user
+  belongs_to :diary_entry
+
+  validates_presence_of :body
+  validates_associated :diary_entry
+end

app/models/diary_entry.rb

@@ -1,5 +1,6 @@
 class DiaryEntry < ActiveRecord::Base
   belongs_to :user
+  has_many :diary_comments, :order => "id"
 
   validates_presence_of :title, :body
   validates_numericality_of :latitude, :allow_nil => true