Can't use :references in add_column

I have a User and an Address model, where User has_one :address. When creating the Address mode, I am planning for a polymorphic association for another model that also has_one :address. I need to use :references in a migration using add_column, just as I would using create_table.

I expected the following to work:

add_column :users, :addressable, :references, :polymorphic => true

here is the error:

== 20080526144501 CreateAddresses: migrating ==================================

-- create_table(:addresses)

-> 0.0040s

-- add_column(:users, :addressable, :references, {:polymorphic=>true})

rake aborted!

Workaround (reversed from schema.rb):

add_column :users, :addressable_id, :integer, :limit => 11

add_column :users, :addressable_type, :string

Thanks,

Mark