* Denormalized post versions. Testing has shown it reduces the size

of the table 66%.

app/models/post_history.rb

@@ -0,0 +1,40 @@
+class PostHistory < ActiveRecord::Base
+  class Error < Exception ; end
+
+  before_validation :initialize_revisions, :on => :create
+  belongs_to :post
+  
+  def self.build_revision_for_post(post)
+    hash = {
+      :source => post.source,
+      :rating => post.rating,
+      :tag_string => post.tag_string,
+      :parent_id => post.parent_id,
+      :user_id => CurrentUser.id,
+      :ip_addr => CurrentUser.ip_addr,
+      :updated_at => revision_time
+    }
+  end
+  
+  def self.revision_time
+    Time.now
+  end
+  
+  def initialize_revisions
+    write_attribute(:revisions, "[]")
+  end
+  
+  def revisions
+    if read_attribute(:revisions).blank?
+      []
+    else
+      JSON.parse(read_attribute(:revisions))
+    end
+  end
+  
+  def <<(post)
+    revision = self.class.build_revision_for_post(post)
+    write_attribute(:revisions, (revisions << revision).to_json)
+    save
+  end
+end