Simple No Overlapping Validation

no_overlapping_validation.rb

class NoOverlappingValidator < ActiveModel::Validator
  def validate(record)
    message = options[:message] || "Record is invalid"
    start = record.read_attribute(options[:start])
    finish = record.read_attribute(options[:finish])
    klass = record.class
    t = klass.arel_table
    c1 = t[:start].lt(finish).and(t[:start].gteq(start))
    c2 = t[:finish].gt(start).and(t[:finish].lteq(finish))
    c3 = t[:start].lteq(start).and(t[:finish].gteq(finish))
    c4 = t[:start].gt(start).and(t[:finish].lt(finish))
    c = (c1.or(c2.or(c3.or(c4)))).and(t[:id].not_eq(record.id))
    overlapped_items = klass.where(c)
    if options[:scope]
      scope_value = record.read_attribute(options[:scope])
      overlapped_items = overlapped_items.where(options[:scope] => scope_value)
    end
    record.errors[:base] = message unless overlapped_items.empty?
  end

  module ClassMethods
    def validates_no_overlapped(*args)
      validates_with NoOverlappingValidator, {:start => args[0], :finish => args[1]}.merge(args[2])
    end
  end
end

timeslot.rb

class Timeslot < ActiveRecord::Base
  extend NoOverlappingValidator::ClassMethods

  belongs_to :venue

  # - Validations
  validates :start, :presence => true
  validates :finish, :presence => true
  validates_associated :venue
  validates_no_overlapped :start, :finish, :scope => :venue_id,
                          :message => "Timeslot is already filled!"

  # This also works:
  # validates_with NoOverlappingValidator, :start => :start, :finish => :finish,
  #  :scope => :venue_id, :message => "Timeslot is already filled!"
end