Auto-creating & updating child element in has_many relationship

channel.rb

class Channel < ActiveRecord::Base
  attr_accessible :private, :title, :color_code, :subscriptions_attributes
  validates_presence_of :user, :title

  after_save :add_owner

  belongs_to :user
  has_many :subscriptions, :inverse_of => :channel, :dependent => :destroy
  has_many :users, :through => :subscriptions, :uniq => true
  accepts_nested_attributes_for :subscriptions, :allow_destroy => true

private

  def add_owner
    owner_subscription = self.subscriptions.find_or_create_by_user_id(user_id)
    owner_subscription.update_attributes!(:accepted => true, :administrator => true)
  end

end

subscription.rb

class Subscription < ActiveRecord::Base
  validates_presence_of :user, :channel
  validates_uniqueness_of :user_id, :scope => [:channel_id], :message => 'user is already subscribed to this channel'

  before_validation :set_user

  belongs_to :channel, :inverse_of => :subscriptions
  belongs_to :user

private

  def set_user
    if user_username.blank?
      self.user_username = user.username
    elsif user_id.blank?
      self.user = User.find_by_username(user_username)
    end
  end

end