When you have a table with serialized columns active record will always treat them as dirty. It can create unnecessary overhead when just incrementing a simple counter.
Fortunately rails has a method that just update the counter and nothing else.
If you have a Movie model with a counter named like_count and you would like to increment it by one you would use the following method.
Movie.update_counters id, :like_count => 1
As a bonus you also get a locking