migration.rb

create_table :users, id: false do |t|
  t.uuid :id, primary: true, null: false

  # ...
end

uuid_primary_key.rb

require 'uuid'

module UUIDPrimaryKey
  extend ActiveSupport::Concern

  included do |klass|
    klass.primary_key = :id
    klass.before_create do
      self.id = UUID.generate
    end
  end

end