カテゴリ単位での所有者の一括変更

[JammyDodger]

I think you need to do the first bit first:

@tshenry

Specify the new owner and categories to operate on. The categories should be an array of category ids, categories 1, 2 and 3 in the example:

rails c
user = User.find_by(username_lower: "lowercase-username")
categories = [1, 2, 3]