Empty CSV file from rails

I am new to ruby on rails and I am having trouble in exporting the data to csv. I have followed the following video:

https://medium.com/coderaga/rails-import-export-csv-data-without-gem-fbf2a36a84f6

I have put require 'csv' in application.rb file also.

My student.rb model has:

def self.to_csv(fields = column_names, options = {})
    CSV.generate(options) do |csv|
      csv << fields
      all.each do |student|
        csv << student.attributes.values_at(*fields)
      end
    end
end

My students_controller has following:

    def index
    @students=Student.all
    puts @students
    respond_to do |format|
      format.html
      format.csv{send_data @students.to_csv(['UIN', 'Name' ,'Section', 'Attempts', 'Score'])}
    end
  end

I think the all.each statement is empty as when I am doing puts student.attributes.values_at(*fields), there is nothing so that is why there is nothing being written. But I am not sure how can I fix it.

Student Load (0.3ms)  SELECT "students".* FROM "students"
#<Student:0x007f9a2386ddb8>
#<Student:0x007f9a2386dc78>
#<Student:0x007f9a2386db38>
  CACHE Student Load (0.0ms)  SELECT "students".* FROM "students"

Please help