class User
  has_many :comments
end

class Comment
  belongs_to :user
  named_scope :recent, :order => 'comments.created_at DESC'
  named_scope :limit, lambda { |limit| {:limit => limit}}
  named_scope :by_unique_users
end

what would I put in the :by_unique_users so that I can do Comment.recent.by_unique_users.limit(3), and only get one comment per user

on sqlite named_scope :by_unique_user, :group => "user_id" works,

but makes it freak out on postgres, which is deployed on production PGError: ERROR: column "comments.id" must appear in the GROUP BY clause or be used in an aggregate function