diff --git a/app/controllers/main_controller.rb b/app/controllers/main_controller.rb
--- a/app/controllers/main_controller.rb
+++ b/app/controllers/main_controller.rb
@@ -57,8 +57,12 @@
     @prob_submissions = Array.new
     @user = User.find(session[:user_id])
     @problems.each do |p|
-      c, sub = Submission.find_by_user_and_problem(@user.id,p.id)
-      @prob_submissions << { :count => c, :submission => sub }
+      sub = Submission.find_last_by_user_and_problem(@user.id,p.id)
+      if sub!=nil
+        @prob_submissions << { :count => sub.number, :submission => sub }
+      else
+        @prob_submissions << { :count => 0, :submission => nil }
+      end
     end
   end
 
diff --git a/app/controllers/problems_controller.rb b/app/controllers/problems_controller.rb
--- a/app/controllers/problems_controller.rb
+++ b/app/controllers/problems_controller.rb
@@ -65,6 +65,6 @@
 
   def stat
     @problem = Problem.find(params[:id])
-    @submissions = Submission.find_last_by_problem(params[:id])
+    @submissions = Submission.find_all_last_by_problem(params[:id])
   end
 end
diff --git a/app/controllers/user_admin_controller.rb b/app/controllers/user_admin_controller.rb
--- a/app/controllers/user_admin_controller.rb
+++ b/app/controllers/user_admin_controller.rb
@@ -77,8 +77,8 @@
       ustat = Array.new
       ustat[0] = u.login
       @problems.each do |p|
-	c, sub = Submission.find_by_user_and_problem(u.id,p.id)
-	if (c!=0) and (sub.points!=nil) 
+	sub = Submission.find_last_by_user_and_problem(u.id,p.id)
+	if (sub!=nil) and (sub.points!=nil) 
 	  ustat << [sub.points, (sub.points>=p.full_score)]
 	else
 	  ustat << [0,false]
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
diff --git a/db/migrate/018_add_number_to_submissions.rb b/db/migrate/018_add_number_to_submissions.rb
new file mode 100644
--- /dev/null
+++ b/db/migrate/018_add_number_to_submissions.rb
@@ -0,0 +1,33 @@
+class AddNumberToSubmissions < ActiveRecord::Migration
+  def self.up
+    add_column :submissions, :number, :integer
+
+    # add number field for all records
+    Submission.reset_column_information
+
+    last_user_id = nil
+    last_problem_id = nil
+    current_number = 0
+
+    Submission.find(:all, 
+                    :order => 'user_id, problem_id, submitted_at').each do |submission|
+      if submission.user_id==last_user_id and submission.problem_id==last_problem_id
+        current_number += 1
+      else
+        current_number = 1
+      end
+      submission.number = current_number
+      submission.save
+
+      last_user_id = submission.user_id
+      last_problem_id = submission.problem_id
+    end
+
+    add_index :submissions, [:user_id, :problem_id, :number], :unique => true
+  end
+
+  def self.down
+    remove_index :submissions, :column => [:user_id, :problem_id, :number]
+    remove_column :submissions, :number
+  end
+end
diff --git a/db/schema.rb b/db/schema.rb
--- a/db/schema.rb
+++ b/db/schema.rb
@@ -9,7 +9,7 @@
 #
 # It's strongly recommended to check this file into your version control system.
 
-ActiveRecord::Schema.define(:version => 17) do
+ActiveRecord::Schema.define(:version => 18) do
 
   create_table "grader_processes", :force => true do |t|
     t.string   "host",       :limit => 20
@@ -83,8 +83,10 @@
     t.datetime "graded_at"
     t.integer  "points"
     t.text     "grader_comment"
+    t.integer  "number"
   end
 
+  add_index "submissions", ["user_id", "problem_id", "number"], :name => "index_submissions_on_user_id_and_problem_id_and_number", :unique => true
   add_index "submissions", ["user_id", "problem_id"], :name => "index_submissions_on_user_id_and_problem_id"
 
   create_table "tasks", :force => true do |t|