gistfile1.rb

# changedの振る舞い
t = Event.new
param = {:year=>2000, :month=>'', :day=>nil}
t.attributes = param

t.changed?       #=> true
t.year_changed?  #=> true
t.month_changed? #=> false
t.day_changed?   #=> false
t.changes        #=> {"year"=>[nil, 2000]}

#============================================

Event.all.each do |event|
  param = params.detect {|id,val| id == event.id}.last
  event.attributes = param
  event.save! if event.changed?
end