rails3 bigint primary key

I would like to create a bigint (or string or whatever that is not int) typed primary key field under Rails 3.

I have a given structure of data, for example:

things
------
id bigint primary_key
name char(32)

The approach I'm currently trying to push:

create_table :things, :id => false do |t| # That prevents the creation of (id int) PK
  t.integer :id, :limit => 8 # That makes the column type bigint
  t.string :name, :limit => 32
  t.primary_key :id # This is perfectly ignored :-(
end

The column type will be correct, but the primary key option will not be present with sqlite3 and I suspect that this is the case for MySQL too.