migration foreign key

Hi All,

I am trying add a foreign key using db:migrate and getting error. I
didn't any documentation on the syntax, if some one has a link, please
send it to me if not, please have a look at the following and tell me
what am I doing wrong?

class name < ActiveRecord:Migration
   def self.up
      create_table :master ...
      end

      add_index :fav, ["lid"], :name => "fk_lid"

      create_table:child ....
      end

      add_constraint "fk_lid" foreign_key ( "lid" ) references :child
("id")
  end

  def self.down
      drop_table :child
      drop_table :master
   end
end

Here is the link that had a good example
http://www.robbyonrails.com/articles/2005/11/11/rails-migrations-and-postgresql-constraints