Først i din terminal:
rails g migration change_date_format_in_my_table
Derefter i din migrationsfil:
For skinner>=3.2:
class ChangeDateFormatInMyTable < ActiveRecord::Migration
def up
change_column :my_table, :my_column, :datetime
end
def down
change_column :my_table, :my_column, :date
end
end