Mongoig Query for product ideas

scope :unrated_by, lambda {|user_id, limit|
	# First, get a list of all of the products a user has already rated.
	ids_to_exclude = Rating.only(:rateable_id).where(:user_id => user_id).collect{|r| r.rateable_id}
	
	# I want to put the user in at a random point in the database to start rating products, 
	# to ensure that all products will have roughly the same number of ratings.  Understanding the 
	# formula below isn't critical to understanding my problem.
	total_idea_pool = GiftIdea.where(:sales_rank.lt => 1200).count - ids_to_exclude.size - limit
	skip = total_idea_pool > 0 ? rand(total_idea_pool) : 0
	
	# now load the products
	GiftIdea:where => {:sales_rank.lt => 1200}.not_in(:_id => ids_to_exclude).skip(skip).limit(limit)
}