Within the current implementation with root ordering there is a problem described in the readme

With numeric ordering, root nodes are, by default, assigned order values globally across the whole database table.

Also, changing the order of the root nodes is quite problematic since they are ordered globally as mentioned earlier.
Typical situation - I want to get all root blocks of a user and change their order:

block = Block.find(id)
old_order_block = block.siblings.where(user_id: user.id).find_by!(sort_order: sort_order)
old_order_block.prepend_sibling(block)

But in this case, the sort_order for all root nodes of one user will be a sparse sequence of 1, 10, 48 ....

I was thinking that in the case where parent_id = nil, you could rely on associations to restrict sort_order to root nodes within the association scope.

My option to implement this as simply as possible is to add an order_belong_to option, through which the foreign key field for the corresponding association is provided.

class Block < ApplicationRecord
 acts_as_tree numeric_order: true, order_belong_to: :user_id
end
class User < ApplicationRecord
end