Validates_uniquenes_of with decimal columns and mysql

I’m using this validation in my model:

validates_uniqueness_of :zoom_level,
:scope => [:latitude, :longitude]

:latitude and :longitude are decimal columns in a mysql database. It
isn’t working. I’ve searched in Google and I found that there were an
error with MySQL but it’s supposed to be fixed.

I’m using Rails 2.3.4. Is there a Rails solution to this problem or
the only solution is creating unique indexes in the database?