Add timestamps to an existing table

I need to add timestamps (created_at & updated_at) to an existing table. I tried the following code but it didn't work.

class AddTimestampsToUser < ActiveRecord::Migration
    def change_table
        add_timestamps(:users)
    end
end

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.

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.

@user1899434's response picked up on the fact that an "existing" table here could mean a table with records already in it, records that you might not want to drop. So when you add timestamps with null: false, which is the default and often desirable, those existing records are all invalid.

But I think that answer can be improved upon, by combining the two steps into one migration, as well as using the more semantic add_timestamps method:

def change
  add_timestamps :projects, default: Time.zone.now
  change_column_default :projects, :created_at, nil
  change_column_default :projects, :updated_at, nil
end

You could substitute some other timestamp for DateTime.now, like if you wanted preexisting records to be created/updated at the dawn of time instead.