@user_answer needs model association (http://rails-bestpractices.com/posts/2-use-model-association)

model_association.rb

begin
  @user_answer = @user.user_answers.where(:question_id => @question_id).first
  if @user_answer
    @user_answer.answer_id = @answer_id
    @user_answer.save!
    @message = 'Answer updated successfully!'
  else
    @user.user_answers.create(:question_id => params[:questionId], :answer_id => params[:answerId])
    @message = 'Answer submitted successfully!'
  end
rescue ex
  @message = ex.message.split(':')[1].chomp
end