Added messaging feature

app/models/message.rb

@@ -0,0 +1,20 @@
+class Message < ActiveRecord::Base
+
+  belongs_to :user_sender, class_name: "User", foreign_key: "user_sender_id"
+  belongs_to :user_target, class_name: "User", foreign_key: "user_target_id"
+
+  validates_presence_of :user_sender, :user_target, :text, on: :create
+
+  def sender
+    @sender ||= if self.user_sender.present?
+      user_sender
+    else
+      User.first
+    end
+  end
+
+  def target
+    # can be nil
+    @target ||= user_target
+  end
+end