Rails has very nice features to validate a model (directly mapped on your database). However it gets sometimes sloppy if you don't know the exact command to do your validation.Here's a trick I had a hard time to find.

Everybody knows how to validate the uniqueness of a field in a database :

validates_uniqueness_of :date

If you now want to validate the uniqueness of a couple (ex: date,user_id) here's how you can:

validates_uniqueness_of :date, :scope => :user_id