diff --git a/app/controllers/exercise_controller.rb b/app/controllers/exercise_controller.rb
index 202752b7..56c8c4ed 100755
--- a/app/controllers/exercise_controller.rb
+++ b/app/controllers/exercise_controller.rb
@@ -76,28 +76,30 @@ class ExerciseController < ApplicationController
     @is_teacher = User.current.allowed_to?(:as_teacher,@course) || User.current.admin?
 
     unless @is_teacher
-      @exercise_user = ExerciseUser.where("user_id=? and exercise_id=?", User.current.id, @exercise.id).first
-      if @exercise_user.nil?
-        @exercise_user = ExerciseUser.create(:user_id => User.current.id, :exercise_id => @exercise.id, :start_at => Time.now, :status => false, :commit_status => 0)
-        if @exercise.exercise_level_settings.where("num != exercise_questions_count").count > 0
-          @exercise.create_user_question_list(@exercise_user.id)
-        end
-        # @exercise_user = ExerciseUser.where("user_id=? and exercise_id=?", User.current.id, @exercise.id).first
-      elsif @exercise_user.start_at.nil?
-        if @exercise.exercise_level_settings.where("num != exercise_questions_count").count > 0
-          @exercise.create_user_question_list(@exercise_user.id)
+      ActiveRecord::Base.transaction do
+        @exercise_user = ExerciseUser.where("user_id=? and exercise_id=?", User.current.id, @exercise.id).first
+        if @exercise_user.nil?
+          @exercise_user = ExerciseUser.create(:user_id => User.current.id, :exercise_id => @exercise.id, :start_at => Time.now, :status => false, :commit_status => 0)
+          if @exercise.exercise_level_settings.where("num != exercise_questions_count").size > 0
+            @exercise.create_user_question_list(@exercise_user.id)
+          end
+          # @exercise_user = ExerciseUser.where("user_id=? and exercise_id=?", User.current.id, @exercise.id).first
+        elsif @exercise_user.start_at.nil?
+          if @exercise.exercise_level_settings.where("num != exercise_questions_count").size > 0
+            @exercise.create_user_question_list(@exercise_user.id)
+          end
+          @exercise_user.update_attributes(:start_at => Time.now)
         end
-        @exercise_user.update_attributes(:start_at => Time.now)
-      end
 
-      @can_edit_excercise = can_edit_exercise @exercise, @exercise_user
-      if !@can_edit_excercise && !@exercise_user.status
-        time = (@exercise_user.start_at.to_i + @exercise.time.to_i * 60) > @exercise.end_time.to_i ? @exercise.end_time : Time.at(@exercise_user.start_at.to_i + @exercise.time.to_i * 60)
-        @exercise_user.update_attributes(:status => true, :end_at => time, :commit_status => 1)
-        s_score = calculate_student_score(@exercise, @exercise_user.user)
-        @exercise_user.update_attributes(:objective_score => s_score, :score => (s_score + (@exercise_user.subjective_score && @exercise_user.subjective_score > 0 ? @exercise_user.subjective_score : 0)))
-        if @exercise_user.user.exercise_answer.where(:exercise_question_id => @exercise.exercise_questions.where(:question_type => 4).map(&:id)).empty?
-          @exercise_user.update_attributes(:subjective_score => 0)
+        @can_edit_excercise = can_edit_exercise @exercise, @exercise_user
+        if !@can_edit_excercise && !@exercise_user.status
+          time = (@exercise_user.start_at.to_i + @exercise.time.to_i * 60) > @exercise.end_time.to_i ? @exercise.end_time : Time.at(@exercise_user.start_at.to_i + @exercise.time.to_i * 60)
+          @exercise_user.update_attributes(:status => true, :end_at => time, :commit_status => 1)
+          s_score = calculate_student_score(@exercise, @exercise_user.user)
+          @exercise_user.update_attributes(:objective_score => s_score, :score => (s_score + (@exercise_user.subjective_score && @exercise_user.subjective_score > 0 ? @exercise_user.subjective_score : 0)))
+          if @exercise_user.user.exercise_answer.where(:exercise_question_id => @exercise.exercise_questions.where(:question_type => 4).map(&:id)).empty?
+            @exercise_user.update_attributes(:subjective_score => 0)
+          end
         end
       end
 
diff --git a/app/models/exercise_user_question.rb b/app/models/exercise_user_question.rb
index 136a187f..ab8c5f84 100755
--- a/app/models/exercise_user_question.rb
+++ b/app/models/exercise_user_question.rb
@@ -3,4 +3,6 @@ class ExerciseUserQuestion < ActiveRecord::Base
   belongs_to :exercise_user
   belongs_to :exercise_question
   # attr_accessible :title, :body
+
+  validates_uniqueness_of :exercise_user_id, :scope => [:exercise_question_id]
 end
diff --git a/db/migrate/20190506070751_migrate_exercise_user_questions.rb b/db/migrate/20190506070751_migrate_exercise_user_questions.rb
deleted file mode 100644
index 3417cf67..00000000
--- a/db/migrate/20190506070751_migrate_exercise_user_questions.rb
+++ /dev/null
@@ -1,30 +0,0 @@
-class MigrateExerciseUserQuestions < ActiveRecord::Migration
-  def up
-    exercise = Exercise.where(:id => 1031).first
-    if exercise.present?
-      exercise.exercise_users.each do |exercise_user|
-        # 分配的实训题数量多于6个的（撤销发布过的学生会有12个题）则删除前六个题
-        if exercise_user.exercise_user_questions.where(:exercise_id => exercise.id).count > 6
-          exercise_user.exercise_user_questions[0..5].each do |question|
-            question.destroy
-          end
-        end
-
-        # 分数超过一百分的需要重新计算得分
-        if exercise_user.objective_score > 100.0
-          score = 0
-          ExerciseUserQuestion.where(exercise_user_id: exercise_user.id, exercise_id: exercise.id).each do |question|
-            question.exercise_shixun_challenges.each do |exercise_cha|
-              score += exercise_cha.exercise_shixun_answers.where(user_id => exercise_user.user_id).try(:score).to_f
-            end
-          end
-
-          exercise_user.update_attributes(score: score, objective_score: score)
-        end
-      end
-    end
-  end
-
-  def down
-  end
-end
diff --git a/db/migrate/20190515070751_migrate_exercise_user_questions.rb b/db/migrate/20190515070751_migrate_exercise_user_questions.rb
new file mode 100644
index 00000000..75f3c29b
--- /dev/null
+++ b/db/migrate/20190515070751_migrate_exercise_user_questions.rb
@@ -0,0 +1,23 @@
+class MigrateExerciseUserQuestions < ActiveRecord::Migration
+  def up
+    exercises = Exercise.where("id > 900")
+
+    exercises.each do |exercise|
+      if exercise.exercise_level_settings.where("num != exercise_questions_count").size > 0
+        exercise.exercise_users.each do |exercise_user|
+          user_question_count = exercise_user.exercise_user_questions.size
+          if user_question_count > exercise.questions_count
+            exercise_user.exercise_user_questions.
+                where(id: exercise_user.exercise_user_questions[exercise.questions_count...user_question_count].map(&:id)).delete_all
+          end
+        end
+      end
+    end
+
+    add_index :exercise_user_questions, [:exercise_user_id, :exercise_question_id], unique: true,
+              name: "index_exercise_user_id_and_exercise_question_id"
+  end
+
+  def down
+  end
+end