gistfile1.rb

class Blog < ActiveRecord::Base
  has_many :posts
  accepts_nested_attributes_for :posts, :allow_destroy => true

  before_validation :set_self_as_parent

  def set_self_as_parent
    self.posts.each |post|
      post.blog = self
    end
  end
end

class Post < ActiveRecord::Base
  belongs_to :blog
  
  validates_presence_of :blog
end


a=Blog.new
a.posts_attributes={0 => {:text => "my cool post"}}
a.save
a.errors.full_messages => "Posts blog can't be blank"