task.rb

class Task < ActiveRecord::Base
  belongs_to :category
  belongs_to :project

  validates_presence_of :project_id, :category_id
  validates_with OvertimeValidator

  scope :of_date, lambda { |date| where(:date => date) }

end


  class OvertimeValidator < ActiveModel::Validator
    def validate(record)
      record.errors[:overtime] << "No more than 8 hours a day, please" if Task.of_date(self.date).map(&:amount).sum + self.amount > 8
    end
  end