Bulk changing ownership on category base

[Festinger]

I found it here:

Administrative Bulk Operations

Below you will find a collection of bulk operations that can be initiated from the command line. You will need SSH access, so if you are a hosted customer, you will need to contact the Discourse team about running these commands. This topic is a wiki, so feel free to add or improve anything relevant. Before working with the console it is extremely important that you have a recent backup. Mistakes can always happen! First thing to do is enter your site’s container: cd /var/discou…

I am getting this error:

[1] pry(main)> topics = Topic.where(category_id: categories).pluck(:id)

topics.each do |topic|
  PostOwnerChanger.new(
    post_ids: Post.where(topic_id: topic).where(post_number: 1).pluck(:id),
    topic_id: topic,
[1] pry(main)> topics = Topic.where(category_id: categories).pluck(:id)
topics.each do |topic|urse.system_user,
  PostOwnerChanger.new(
    post_ids: Post.where(topic_id: topic).where(post_number: 1).pluck(:id),
    topic_id: topic,
    new_owner: user,
    acting_user: Discourse.system_user,
    skip_revision: true
  ).change_owner!
NameError: undefined local variable or method `categories' for main:Object
from (pry):1:in `__pry__'
[2] pry(main)>
[3] pry(main)>