result_sets_controller.rb

class ResultSetsController < ApplicationController
  def index
    @survey = Survey.find(params[:id]) # should be :survey_id if you want to conform to rails standard.
    @quiz = @survey.quiz # try to retrieve quiz from survey, you should not have to load all the result sets to find the right quiz id.
    @result_sets = Survey.result_sets.owned_by(result_set_owner)
    # Rest of code follows below but requires major refactoring.
  end
  
  private
  
  def result_set_owner
    if current.loginable_type == 'Student'
      params[:user_id] || current_user.id
    else
      params[:user_id]
    end
  end
  
end

survey.rb

class ResultSet < ActiveRecord::Base
  def self.owned_by(owner_id)
    return self if owner_id.nil?
    where(user_id: owner_id)
  end
end