Rails - Jan 2006 - Multi Row validation... or running queries within Validate function?

Is there anyway to perform a query from within a validate function?

I need to verify that the sum of several rows does not exceed a certain 
value eg:

class Entry < ActiveRecord::Base
        belongs_to :day
        belongs_to :project
 
        def validate
                totalhours = hours
                samedayentries = self.find_by_day_id(day_id)
 
                samedayentries.each do |i|
                               totalhours += i[:hours] unless (id == i[:id])
                end
                errors.add_to_base( "You cannot enter more than 24 hours 
in a day.") if totalhours > 24
        end
end

Or some better way to accomplish the same general idea.  (I can do it in 
the controller, but it really belongs in the model IMHO.)

Thanks
Mark Farver