Rails - May 2005 - acts_as_tree / moving children

Greetings!

I am working on a model that utilizes "acts_as_tree".  I have a table 
named ''categories'' that has multiple root items, each each
root item
has several children.

I cannot successfully separate those children from the tree.  For 
example, if I delete a root category, I want the sub categories to 
become roots.  I have tried setting the ''parent'' attribute of
the
children to ''nil'', but that isn''t making any
difference.

Does anyone have any recommendations?

Thank you,

Michael