Add timestamps to an existing table

Migrations are just two class methods (or instance methods in 3.1): up and down (and sometimes a change instance method in 3.1). You want your changes to go into the up method:

class AddTimestampsToUser < ActiveRecord::Migration
  def self.up # Or `def up` in 3.1
    change_table :users do |t|
      t.timestamps
    end
  end
  def self.down # Or `def down` in 3.1
    remove_column :users, :created_at
    remove_column :users, :updated_at
  end
end

If you're in 3.1 then you could also use change (thanks Dave):

class AddTimestampsToUser < ActiveRecord::Migration
  def change
    change_table(:users) { |t| t.timestamps }
  end
end

Perhaps you're confusing def change, def change_table, and change_table.

See the migration guide for further details.

The timestamp helper is only available in the create_table block. You can add these columns by specifying the column types manually:

class AddTimestampsToUser < ActiveRecord::Migration
  def change_table
    add_column :users, :created_at, :datetime, null: false
    add_column :users, :updated_at, :datetime, null: false
  end
end

While this does not have the same terse syntax as the add_timestamps method you have specified above, Rails will still treat these columns as timestamp columns, and update the values normally.