extended best_sellers to take a date range as args

defaults to start one week ago, finish now

app/models/promotions.rb

@@ -8,12 +8,13 @@ def self.random_subset(n)
     promos.map {|p| p.product }
   end 
 
-  def self.best_sellers(n)
+  def self.best_sellers(n, start = 1.week.ago, finish = Time.now)
     # most ordered items
-    # looks for completed orders from past week, then gets top N sellers by quantity
+    # looks for completed orders from date range (default: the past week)
+    # then gets top N sellers by quantity
     # query: need to filter out cancelled orders? 
     best_n = Order.checkout_completed(true).
-                   between(1.week.ago,Time.now).
+                   between(start,finish).
                    find(:all, 
                         :joins => "INNER JOIN line_items ON orders.id = line_items.order_id", 
                         :select => "variant_id, SUM (quantity) sum",