In Rails 2.1, they added partial updates - which is a great idea and a long time coming. This makes ActiveRecord only updates the parts of a model that has actually changed, and also gives you the ability to see if a field (or a record) has changed since retrieving it from the database. (via model.changed?, model.attribute_changed?, and friends)
There is a gotcha though, If you are using serialized attributes it won't trigger the changed flag unless you assign to it - so if you are doing something like:
model.something_serialized[:foo] = true
ActiveRecord will not realize you have changed anything and it won't get saved. You can get around this by doing something like:
model.something_serialized = model.something_serialized.merge( :foo => true )
