link.rb

class Link < ActiveRecord::Base
  acts_as_revisable
  belongs_to :post
end

link_revision.rb

class LinkRevision < ActiveRecord::Base
  acts_as_revision
  belongs_to :post, :class_name => "PostRevision"
  before_create :reassociate_with_post, :if => :post_in_revision?
  
  private
    def reassociate_with_post
      self.post = self.current_revision.post.find_revision(:previous)
    end
    
    def post_in_revision?
      self.current_revision.post.in_revision?
    end
end

post.rb

class Post < ActiveRecord::Base
  acts_as_revisable
  has_many :links, :before_add => :revise!
  after_revise :revise_links!
  
  private
    def revise_links!
      self.links.each(&:revise!)
    end
end

post_revision.rb

class PostRevision < ActiveRecord::Base
  acts_as_revision
  has_many :links, :class_name => "LinkRevision", :foreign_key => "post_id"
end