How to make Active Record join return unique objects?

@dbjohn has the right idea, but I assume you want to avoid creating extra objects. Here's a slight variant on his solution, letting the database do the uniq-ing for you:

date = "2013-01-01 00:00:00"
User.joins(:orders).where("orders.created_at >= ?", date).distinct

Note that you can rearrange the order of methods to fit whatever you think is most semantic, and ActiveRecord will write the same SQL for you.

User.joins(:orders).
    where("orders.created_at >= '2013-01-01 00:00:00'").
    group('users.id')

group method will chain to the query and give you a list of unique records.