diff --git a/app/models/message.rb b/app/models/message.rb
new file mode 100644
--- /dev/null
+++ b/app/models/message.rb
@@ -0,0 +1,60 @@
+class Message < ActiveRecord::Base
+
+  belongs_to :sender, :class_name => "User"
+  belongs_to :receiver, :class_name => "User"
+
+  belongs_to :replying_message, :class_name => "Message"
+
+  # commented manually do it
+  #
+  #has_many :replied_messages, {
+  #  :class_name => "Message", 
+  #  :foreign_key => "replying_message_id"
+  #}
+  #
+
+  attr_accessor :replied_messages
+
+  def self.find_all_sent_by_user(user)
+    messages = user.messages
+    replied_messages = user.replied_messages
+    Message.build_replying_message_hierarchy messages, replied_messages
+    return messages
+  end
+  
+  def self.find_all_system_unreplied_messages
+    self.find(:all, 
+              :conditions => 'ISNULL(receiver_id) ' + 
+                             'AND (ISNULL(replied) OR replied=0)',
+              :order => 'created_at')
+  end
+
+  def self.build_replying_message_hierarchy(*args)
+    # manually build replies hierarchy (to improve efficiency)
+    all_messages = {}
+
+    args.each do |collection|
+      collection.each do |m| 
+        all_messages[m.id] = m
+        m.replied_messages = []
+      end
+    end
+
+    all_messages.each_value do |m|
+      rep_id = m.replying_message_id
+      if all_messages[rep_id]!=nil
+        all_messages[rep_id].add_replied_message(m)
+      end
+    end
+  end
+
+  def add_replied_message(m)
+    if @replied_messages==nil
+      @replied_messages = [m]
+    else
+      @replied_messages << m
+    end
+    @replied_messages
+  end
+
+end