Update one column to value of another in Rails migration

I have a table in a Rails app with hundreds of thousands of records, and they only have a created_at timestamp. I'm adding the ability to edit these records, so I want to add an updated_at timestamp to the table. In my migration to add the column, I want to update all rows to have the new updated_at match the old created_at, since that's the default for newly created rows in Rails. I could do a find(:all) and iterate through the records, but that would take hours because of the size of the table. What I really want to do is:

UPDATE table_name SET updated_at = created_at;

Is there a nicer way to do that in a Rails migration using ActiveRecord rather than executing raw SQL?

I would create a migration

rails g migration set_updated_at_values

and inside it write something like:

class SetUpdatedAt < ActiveRecord::Migration
  def self.up
    Yourmodel.update_all("updated_at=created_at")
  end

  def self.down
  end
end

This way you achieve two things

• this is a repeatable process, with each possible deploy (where needed) it is executed
• this is efficient. I can't think of a more rubyesque solution (that is as efficient).

Note: you could also run raw sql inside a migration, if the query gets too hard to write using activerecord. Just write the following:

Yourmodel.connection.execute("update your_models set ... <complicated query> ...")

You can use update_all which works very similar to raw SQL. That's all options you have.

BTW personally I do not pay that much attention to migrations. Sometimes raw SQL is really best solution. Generally migrations code isn't reused. This is one time action so I don't bother about code purity.