validations.rb

class Host < ActiveRecord::Base
  validates_associated :access_points, :switches
  before_validation :set_group_sizes

  def set_group_sizes
    associated = (self.access_points + self.switches).group_by(&:ip)
    associated.each do |ip, o|
      o.each do |obj|
        obj.devices_on_ip = o.length
      end
    end
  end
end

class Switch < ActiveRecord::Base
  attr_accessor :devices_on_ip
  validates :devices_on_ip, :numericality => {:less_than_or_equal_to => 1, :allow_blank => true}
end

class AccessPoint < ActiveRecord::Base
  attr_accessor :devices_on_ip
  validates :devices_on_ip, :numericality => {:less_than_or_equal_to => 1, :allow_blank => true}
end