how to disable auto-increment in primary key.

How can i disable the auto-increment for a primary key data type. The
purpose of using primary_key data type is using different name for
primary key. But we still want to use all the properties of "id"
except for auto increment part.

I had my migration as following
def self.up
    create_table :pk2s , :id=> false do |t|
      t.primary_key :uid
      t.string :name


# ========== Model
class Pk2 < ActiveRecord::Base
  set_primary_key "uid"