validation method dealing with _destroy attribute from accepts_nested_attributes_for

gistfile1.rb

class Project < ActiveRecord::Base

  has_many :tasks
  accepts_nested_attributes_for :tasks, :reject_if => :all_blank, :allow_destroy => true

  def validate
    # require a minimum of one task
    undestroyed_task_count = 0

    tasks.each { |t| undestroyed_task_count += 1 unless t.marked_for_destruction? }

    errors.add_to_base 'There must be at least one task' if undestroyed_task_count < 1
  end

end