The goal of rich-join-association is to make it simpler to make use of has_many :through associations in which the join table has a single additional attribute.

(Example adapted from http://blog.hasmanythrough.com/articles/2006/04/20/many-to-many-dance-off)
So if you had Dancers, Movies, and Appearances and each Appearance had an attribute character_name, rather than saying:

  dan = Dancer.new
  cameo = Appearance.new
  cameo.dancer = dan
  cameo.movie = Movie.find_by_name 'Served'
  cameo.character_name = 'Joe the Dancemaster'
&nbsp;&nbsp;dan.appearances << cameo

One would simply enter

&nbsp;&nbsp;dan = Dancer.new
&nbsp;&nbsp;s.cameos['Served'] = 'Joe the Dancemaster'

Currently, you can only use one join attribute per pair of classes joined, but if an Appearance also joined a Dancer to a Song with an attribute quality_of_performance, that would work as well.