How to add parent_id to existing instances?

[paddy-hack]

I followed the Installation instructions from the README.md but was confused by the last step. That says

7. If you're migrating from another system where your model already has a parent_id column, run Tag.rebuild! and your tag_hierarchies table will be truncated and rebuilt.

If you're starting from scratch you don't need to call rebuild!.

At this point, my model has a parent_id column but I am not migrating from another system. I am simply adding hierarchy to an existing model, let's call it Tag, and the database has a small pile of instances. The migrations added the parent_id column to that model and closure_tree:migration generator added an empty tag_hierarchies table to the database. All Tag instances have their parent_id attribute set to nil at this point.

If I then execute something like Tag.first.ancestry_path in the Rails console, I get an empty array. I had expected to get an array with Tag.first.name.

Running Tag.rebuild! (before saving any parent_id values to the database) seems to fix this but I am not sure if that is what I am supposed to be doing given the documentation. If it is, is doing this backfilling in a migration the recommended way of doing so? How would I revert that?

[paddy-hack]

FWIW, I settled on using a migration like

class SeedTagHierarchies < ActiveRecord::Migration[8.1]
  def up
    Tag.rebuild!
  end

  def down
    Tag.hierarchy_class.destroy_all
  end
end

Note that within the context of the migration TagHierarchy cannot be used.