How to write your own created_at/updated_at?

I'm using Pfeed( plugin for User
Activity Feeds.
It creates PfeedItem model records and uses created_at field for a lot
of stuff inside.

Now I need to export/import those user logs. I do exporting to YAML.
When I reconstruct PfeedItem AR object from YAML, it have real(old)
creation time in created_at attribute. When I save this PfeedItem -
there is new created_at with current time written to database.

How do I force created_at to be not automatically generated, but
contain my old created_at timestamp from YAML?

Active Record shouldn't overwrite created_at if you set it yourself