json.commit_users_count @exercise_users_size  #已提交用户数
json.exercise_users do
  json.array! @exercise_users do |ex|
    ex_user = ex.user
    json.user_id ex_user.id
    json.user_name ex_user.real_name
    json.student_id ex_user.user_extension.student_id
    json.user_score ex.score
  end
end