Multi Row validation... or running queries within Validate f

Rails
1

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 F.