Ruby on Rails
PostgreSQL Sequences and Rails

SEQUENCES are supported in RAILS via PolymorphicAssociations

And that is all you need to know.

BUT, see also:

• HowToUsePolymorphicAssociations – an example combining :polymorphic and :through
• SimplePolymorphicAssociation – a simpler example using :polymorphic by itself

there was a change in Rails 1.13.2 “PostgreSQL: correctly discover custom primary key sequences.”

some more information:
http://blog.evanweaver.com/articles/2006/06/06/polymorphic-association-helpers
http://www.robbyonrails.com/articles/2005/08/20/postgresql-sequences-in-rails
http://blog.hasmanythrough.com/articles/2006/04/03/polymorphic-through

Dealing with legacy sequences in ActiveRecord:

I had a legacy postgres database that had sequences for creating the primary keys on some of the tables. These tables weren’t setup with the sequences as a default value for the primary key.. the legacy code prefetched the next sequence value then inserted a record with that ID. Looking at active record, it seemed like it should be a snap to reproduce the same functionality.

This is what I ended up with:
(replace the table names “address” and “user_profile” with your own). If you need prefetching on all your tables, just return true from “prefetch_primary_key?”.

module ActiveRecord
  module ConnectionAdapters
    class PostgreSQLAdapter 
      def prefetch_primary_key?(table_name = nil)
        table_name && %w{address user_profile}.member?(table_name)
      end

      def next_sequence_value(sequence_name)
        Integer(select_value("SELECT nextval('#{sequence_name}')"))
      end
    end
  end
end

class UserProfile < ActiveRecord::Base
  set_table_name "user_profile"
  set_primary_key "user_id"
  set_sequence_name "useridsequence"
  belongs_to :address
end


class Address < ActiveRecord::Base
  set_table_name "address"
  set_primary_key "address_id"
  set_sequence_name "addressidsequence"
  has_one :user_profile
end