The one I love most

If a Model class has a "has_many" relationship available, it can specify a "has_one" relationship pointing to one record for future convenient reference.

:conditions and other SQL options can be used to narrow the search.

Blog has many Post

class Blog < ActiveRecord::BASE
  has_many :posts  #has_many must be present
  has_one :first_post,  #no naming convention.
          :class_name => 'Post', #must specify!
          :order => 'created_at asc'
end
 
class Post < ActiveRecord::BASE
  belongs_to :blog   #one belongs_to is enough
end

Later, you can reference the first record easily by:

  >>blog1 = Blog.find :first
  >>blog1.first_post

Yes! this is the one I love most, give her some special treat.