class Product < ActiveRecord::Base
  has_many :announcement_products
  has_many :announcements, :through => :announcement_products

  accepts_nested_attributes_for :announcements, :reject_if => lambda { |a| a[:content].blank? }, :allow_destroy => true
end

class Announcement < ActiveRecord::Base
  has_many :announcement_products
  has_many :products, :through => :announcement_products

  accepts_nested_attributes_for :products, :reject_if => lambda { |a| a[:content].blank? }, :allow_destroy => true
end

class AnnouncementProduct < ActiveRecord::Base
  belongs_to :announcement
  belongs_to :product
end

-------------
CONSOLE

a = Announcement.new()
a.products << Product.first
a.products << Product.last
a.name = 'foo'
a.description = 'bar'
a.save!