gistfile1.rb

class Device
  validates_uniqueness_of :device_number, :scope => :customer_id

  after_create: assign_device_number

  def assign_device_number
    begin 
      device_number = self.customer.devices.map(&:device_number).compact.max) || 0  + 1
      save!
    rescue Whatever gets thrown for validation error
      assign_device_number
  end
end