how should I model this?

I have Users and Tasks.

Each User will complete the same Tasks, and I want to record the date
of completion.

Any suggestions on the best way to model this?

I have Users and Tasks.

Each User will complete the same Tasks, and I want to record the date
of completion.

Sounds like users have many tasks through some join model. An instance
of that join model describes progress that user has made on the
corresponding task

Fred

Use has many through with a new model called todo that will have
completed at datetime field

Http://www.rubyplus.org
Free Ruby & Rails screencasts