Make sure that there is a sender and a recipient of a message. New test to make sure that there a message also has a sender and a recipient, as the validates_associated doesn't do anything if they are nil.

app/models/message.rb

@@ -2,7 +2,7 @@ class Message < ActiveRecord::Base
   belongs_to :sender, :class_name => "User", :foreign_key => :from_user_id
   belongs_to :recipient, :class_name => "User", :foreign_key => :to_user_id
 
-  validates_presence_of :title, :body, :sent_on
+  validates_presence_of :title, :body, :sent_on, :sender, :recipient
   validates_inclusion_of :message_read, :in => [ true, false ]
   validates_associated :sender, :recipient
 end

test/unit/message_test.rb

@@ -20,9 +20,15 @@ class MessageTest < Test::Unit::TestCase
   
   def test_validating_msgs
     message = messages(:one)
-    assert_equal true, message.valid?
+    assert message.valid?
     massage = messages(:two)
-    assert_equal true, message.valid?
+    assert message.valid?
   end
   
+  def test_invalid_send_recipient
+    message = messages(:one)
+    message.sender = nil
+    message.recipient = nil
+    assert !message.valid?
+  end
 end