A plugin to easily add constraints into a PostgreSQL database.  I think
it would be easy to add other database engines to it assuming they
implement constraints.
The GIT repository is here:
http://github.com/pedz/activerecord_constraints
The rdoc is here:
http://pedz.github.com/activerecord_constraints
A small snippet from the readme:
Unique Constraint:
    class CreateFoos < ActiveRecord::Migration
      def self.up
        create_table :foos do |t|
          # name will have a "unique" constraint
          t.string :name, :null => false, :unique => true
        end
      end
    end
Trivial foreign key constraint:
    class CreateFoos < ActiveRecord::Migration
      def self.up
        create_table :foos do |t|
          # bar_id will now be a foreign key constraint column id in
table bar
          t.integer :bar_id, :null => false, :reference => true
        end
      end
    end
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