gistfile1.rb

class Lesson < ActiveRecord::Base
  belongs_to :period
  validates_presence_of :period_id
end

class Period < ActiveRecord::Base
  has_many :lessons, :dependent=>:delete_all
  validates_associated :lessons
end


#in a controller:
...
5.times do
  period.lessons.build(:date=>"..")
end

period.save
...
#could never save!!