Rails - Nov 2009 - 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?