validates :id, :uniqueness => true, :presence => { :on => :update },
:numericality => { :on => :update, :only_integer => true }
- validates :user_id, :presence => true,
- :numericality => { :only_integer => true }
validates :num_changes, :presence => true,
:numericality => { :only_integer => true,
:greater_than_or_equal_to => 0 }