[mysql] Change a column type from Date to DateTime during ROR migration

I need to change my column type from date to datetime for an app I am making. I don't care about the data as its still being developed.

How can I do this?

AFAIK, migrations are there to try to reshape data you care about (i.e. production) when making schema changes. So unless that's wrong, and since he did say he does not care about the data, why not just modify the column type in the original migration from date to datetime and re-run the migration? (Hope you've got tests:)).

Also, if you're using Rails 3 or newer you don't have to use the up and down methods. You can just use change:

class ChangeFormatInMyTable < ActiveRecord::Migration
  def change
    change_column :my_table, :my_column, :my_new_type
  end
end

There's a change_column method, just execute it in your migration with datetime as a new type.

change_column(:my_table, :my_column, :my_new_type)

In Rails 3.2 and Rails 4, Benjamin's popular answer has a slightly different syntax.

First in your terminal:

$ rails g migration change_date_format_in_my_table

Then in your migration file:

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