Find the last entry by user_id

How do I find the all records that are the latest from each user, by
user_id? I'm trying to get an :average working but I can't solve it.
This is my find:

Dada.average(:column_with_nr, :conditions => ["created_at > ?",
@date.to_s(:db)], :order => 'created_at', :select => 'DISTINCT user_id')
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