educoder/db/migrate/20191203040140_add_uniq_index_to_poll_votes.rb

class AddUniqIndexToPollVotes < ActiveRecord::Migration[5.2]
  def change
    # remove_index :poll_votes, column: [:poll_question_id, :user_id]
    #
    # change_column_default :poll_votes, :poll_question_id, from: nil, to: -1
    # PollVote.where(poll_answer_id: nil).update_all(poll_answer_id: -1)
    #
    # sql = %Q(delete from poll_votes where (poll_question_id, user_id, poll_answer_id) in
    #         (select * from (select poll_question_id, user_id, poll_answer_id from poll_votes group by poll_question_id, user_id, poll_answer_id having count(*) > 1) a)
    #         and id not in (select * from (select min(id) from poll_votes group by poll_question_id, user_id, poll_answer_id having count(*) > 1 order by id) b))
    # ActiveRecord::Base.connection.execute sql
    #
    # add_index :poll_votes, [:poll_question_id, :user_id, :poll_answer_id], name: 'poll_answer_index', unique: true

  end
end