db:schema:dump constraints query

‘Create a db/schema.rb file that can be portably used against any DB supported by AR’

This definition I understand fully, but is there a reason why key constraints are not carried across? For instance I have a primary key constraint on a table in my postgreSQL dB as such:

‘ALTER TABLE orderstbl
ADD CONSTRAINT orderstbl_pkey PRIMARY KEY(order_id);’

but in my schema.rb file the table is simply presented as:

create_table “orderstbl”, :id => false, :force => true do |t|

t.column "order_id",                :string,   :null => false
t.column "version",                 :integer,  :null => false
t.column "price_total",             :integer,  :null => false

t.column "order_date",              :datetime, :null => false
t.column "user_id",                 :integer,  :null => false
t.column "user_ip",                 :string

t.column "downloaded",              :boolean,  :null => false
t.column "merchant_id",             :integer,  :null => false
t.column "merchant_transaction_id", :string,   :null => false

t.column "downloaded_date",         :datetime

end

How can I force the schema:dump to carry across at least the primary/foreign key constraints? The process manages to ‘add_index’ but nothing else!!