I have recently been working on Joomla - running among other things an instance of Mosets tree 2.1. I have been working on a custom import from an external database. While you can get Mosets to rebuild its hierarchy using
http://www.yourdomain.com/administrator/index.php?option=com_mtree&task=rebuild_tree
this will not always manipulate your data into a form that Mosets can use.
I ran into trouble until I found that the root element must have an id of 0 and it must have a parent of -1. Any id other than 0 will not fail catastrophically, however it will not work properly either (I was seeing around 5% of the elements in the table showing up).
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