ruby script/plugin install svn://rubyforge.org//var/svn/araddconstraint/trunk

Create an FK to support a default rails has_many / belongs_to

 create_table :posts do |t|
   t.column :title, :string
   t.column :body, :text
 end

 create_table :comments do |t|
   t.column :contents, :text
   t.column :post_id, :integer
 end

 add_foreign_key :comments, :posts

(creates an FK constraint called fk_comments_posts from comments.post_id to posts.id)
Custom source column

 create_table :posts do |t|
   t.column :title, :string
   t.column :body, :text
 end

 create_table :comments do |t|
   t.column :contents, :text
   t.column :my_parent_post_id, :integer
 end

 add_foreign_key :comments, :posts, :source_column => :my_parent_post_id

(creates an FK constraint called fk_comments_posts from comments.my_parent_post_id to posts.id)
Custom key name

 create_table :programmers do |t|
   t.column :name, :string
 end

 create_table :pairs do |t|
   t.column :first_id, :integer
   t.column :second_id, :integer
 end

 add_foreign_key :pairs, :programmers,
         :source_column => :first_id,
         :name => :fk_first_programmer
 add_foreign_key :pairs, :programmers,
         :source_column => :second_id,
         :name => :fk_second_programmer

There is currently no advanced documentation for this plugin.