diff --git a/app/models/submission.rb b/app/models/submission.rb
--- a/app/models/submission.rb
+++ b/app/models/submission.rb
@@ -10,20 +10,17 @@
   validate :must_specify_language
   validate :must_have_valid_problem
 
-  def self.find_by_user_and_problem(user_id, problem_id)
-    subcount = count(:conditions => "user_id = #{user_id} AND problem_id = #{problem_id}")
-    if subcount != 0
-      last_sub = find(:first, 
-		      :conditions => {:user_id => user_id,
-			:problem_id => problem_id},
-		      :order => 'submitted_at DESC')
-    else
-      last_sub = nil
-    end
-    return subcount, last_sub
+  before_save :assign_latest_number
+
+  def self.find_last_by_user_and_problem(user_id, problem_id)
+    last_sub = find(:first, 
+                    :conditions => {:user_id => user_id,
+                      :problem_id => problem_id},
+                    :order => 'submitted_at DESC')
+    return last_sub
   end
 
-  def self.find_last_by_problem(problem_id)
+  def self.find_all_last_by_problem(problem_id)
     # need to put in SQL command, maybe there's a better way
     Submission.find_by_sql("SELECT * FROM submissions " +
 			   "WHERE id = " +
@@ -33,6 +30,8 @@
 			   "GROUP BY user_id)")
   end
 
+  protected
+
   def self.find_option_in_source(option, source)
     if source==nil
       return nil
@@ -92,4 +91,10 @@
     end
   end
 
+  # callbacks
+  def assign_latest_number
+    latest = Submission.find_last_by_user_and_problem(self.user_id, self.problem_id)
+    self.number = (latest==nil) ? 1 : latest.number + 1;
+  end
+
 end