Transforming flat xml to nested xml using Xquery and recursive functions - xml-serialization

Question

I'm troubled with an exercise on recursive functions in Xquery. I need to transform a flat XML tree to a nested XML tree using only Xquery.
The flat XML looks like this:
<?xml version="1.0"?>
<tree>
<node id="1" type="Folder">
<child>2</child>
<child>3</child>
</node>
<node id="2" type="Folder">
<child>4</child>
</node>
<node id="3" type="Folder">
<child>5</child>
<child>6</child>
</node>
<node id="4" type="Item" />
<node id="5" type="Folder">
<child>7</child>
</node>
<node id="6" type="Item" />
<node id="7" type="Item" />
</tree>
The nested XML required looks like this:
<?xml version="1.0"?>
<tree>
<node id="1" type="Folder" children="2 3">
<node id="2" type="Folder">
<node id="4" type="Item" />
</node>
<node id="3" type="Folder">
<node id="5" type="Folder" children="7">
<node id="7" type="Item" />
</node>
<node id="6" type="Item" />
</node>
</node>
</tree>
I've tried to Xquery this without recursive functions, but without much luck. Especially the conditioning seems strange to me; the root element of the new nested XML should be the node with id="1", since it does not exist as a child of any other element. However, if I try to specify this as a condition, e.g. below, it does not seem possible to just select that node.
for $node in /Tree/node[#id != /Tree/node/child]
return
<node id="{data($node/#id)}" type="{data($node/#type)}">
(: Lower level subqueries.... :)
</node>
UPDATE: I've come a lot further, but now I'm stuck with the [condition] on the selection of nodes that have an id equal to the contents of a parent node, i.e. the for statement in the recursive function does not give back any nodes, which is unexpected.
This is my code thus far:
declare namespace output = "http://www.w3.org/2010/xslt-xquery-serialization";
declare namespace local = "localhost";
declare option output:method "xml";
declare option output:omit-xml-declaration "no";
declare option output:indent "yes";
declare option output:doctype-system "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd";
declare function local:addSubNode($n)
{
for $subnode in doc("xml/flat-tree.xml")/tree/node[#id = $n]
let $subnid:=data($subnode/#id)
let $subtype:=data($subnode/#type)
return <node id="{$subnid}" type="{$subtype}">
{local:addSubNode($subnid)}
</node>
};
<tree>
{
for $node in doc("xml/flat-tree.xml")/tree/node[not(#id = /tree/node/child)]
let $nid:=data($node/#id)
return <node id="{$nid}" type="{data($node/#type)}">
{for $child in $node
let $cid:=data($child)
return local:addSubNode($cid)
}
</node>
}
</tree>

Answer 1

Just a partial answer - = and != can compare an atomic value against a sequence, but it works as a logical OR against each term. That is "a = b" means "is there a value in b that equals a" and "a != b" means "is there a value in b that does not equal a", which is not what you're looking for. You want "is there no value in b which equals a" which is "not(a = b)".
for $node in $d/node[not(#id = $d/node/child)] return
<node id="{$node/#id}" type="{$node/#type}">
(: etc. :)
</node>

Answer 2

Ok got it! Eventually I fed the whole $node or $child element to the recursive function instead of just its id attribute, now it works!
declare namespace output = "http://www.w3.org/2010/xslt-xquery-serialization";
declare namespace local = "localhost";
declare option output:method "xml";
declare option output:omit-xml-declaration "no";
declare option output:indent "yes";
declare option output:doctype-system "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd";
(:lookupChildren function :)
declare function local:lookupChildren($p)
{
for $child in $p/child
return $child
};
declare function local:addSubNode($n)
{
for $subnode in doc("xml/flat-tree.xml")/tree/node
let $subnid:=data($subnode/#id)
let $subtype:=data($subnode/#type)
where $subnode/#id = $n/child
return
<node id="{$subnid}" type="{$subtype}" children="{local:lookupChildren($subnode)}">
{local:addSubNode($subnode)}
</node>
};
<tree>
{
for $node in doc("xml/flat-tree.xml")/tree/node[not(#id = /tree/node/child)]
let $nid:=data($node/#id)
let $ntype:=data($node/#type)
return <node id="{$nid}" type="{$ntype}" children="{local:lookupChildren($node)}">
{local:addSubNode($node)}
</node>
}
</tree>