def protect_master_user
  unless User.find_all_by_account_id_and_is_master(self.account_id, true).count > 1
    false
  end
end

class Account < ActiveRecord::Base
  has_many :users

  before_destroy :destroy_users
  protected
    def destroy_users
      users.each do |user|
        u.account_id = nil
        u.destroy
      end
    end

end

class User < ActiveRecord::Base
  belongs_to :account

  before_destroy :protect_master_user

  protected
    def protect_master_user
      unless account_id.nil? ||! master ||
        User.find_all_by_account_id_and_master(self.account_id, true).count > 1
        errors.add_to_base "Cannot remove master user."
        return false
      end
    end
end