class Exam has_many :questions
           has_and_belongs_to_many :users

class Question has_many :answers
               has_many :user_answers
               belongs_to :exam

class Answer belongs_to :question

class UserAnswer belongs_to :question
                 belongs_to :user

class User has_and_belongs_to_many :exams
           has_many :user_answes

answer = Answer.find(params[:id])
            user_answer = UserAnswer.create(answer_id: answer.id, user_id: user.id, question_id: answer.question.id)