Added basic test coverage for a model that invokes for protection from mass assignment.

test/dummy/app/models/protected_widget.rb

@@ -0,0 +1,3 @@
+class ProtectedWidget < Widget
+  attr_accessible :name, :a_text
+end

test/unit/protected_attrs_test.rb

@@ -0,0 +1,41 @@
+require 'test_helper'
+
+class ProtectedAttrsTest < ActiveSupport::TestCase
+  subject { ProtectedWidget.new }
+
+  accessible_attrs = ProtectedWidget.accessible_attributes.to_a
+  accessible_attrs.each do |attr_name|
+    should allow_mass_assignment_of(attr_name.to_sym)
+  end
+  ProtectedWidget.column_names.reject { |column_name| accessible_attrs.include?(column_name) }.each do |attr_name|
+    should_not allow_mass_assignment_of(attr_name.to_sym)
+  end
+
+  context 'A model with `attr_accessible` created' do
+    setup do
+      @widget = ProtectedWidget.create! :name => 'Henry'
+      @initial_attributes = @widget.attributes
+    end
+
+    should 'be `nil` in its previous version' do
+      assert_nil @widget.previous_version
+    end
+
+    context 'which is then updated' do
+      setup do
+        @widget.assign_attributes(:name => 'Jeff', :a_text => 'Short statement')
+        @widget.an_integer = 42
+        @widget.save!
+      end
+
+      should 'not be `nil` in its previous version' do
+        assert_not_nil @widget.previous_version
+      end
+
+      should 'the previous version should contain right attributes' do
+        assert_equal @widget.previous_version.attributes, @initial_attributes
+      end
+    end
+
+  end
+end