You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: 20171017221434_cleanup_empty_signatures.rb
    path of file: ./repos/diaspora/db/migrate
    
    the code of the file until where you have to start completion: # frozen_string_literal: true

class CleanupEmptySignatures < ActiveRecord::Migration[5.1]
  class Comment < Ap
    
  def up
    Comment.joins("INNER JOIN comment_signatures as signature ON comments.id = signature.comment_id")
           .where("signature.author_signature = ''").destroy_all
    Like.joins("INNER JOIN like_signatures as signature ON likes.id = signature.like_id")
        .where("signature.author_signature = ''").destroy_all
    PollParticipation.joins("INNER JOIN poll_participation_signatures as signature " \
                            "ON poll_participations.id = signature.poll_participation_id")
                     .where("signature.author_signature = ''").destroy_all
  end