Can I make XQuery evaluate { $expression } within

问题:

To mimic an auto-increment value in XQuery Update, the following works fine, assuming <root count="0"/> when running this for the first time:

let $count := /root/@count
return (
  insert node <node id='{ $count }'/> into /root,
  replace value of node $count with $count + 1  
)

...nicely yielding:

<root count="1">
  <node id="0">
</root>

However, I'd like to define the node in my Java code, and then bind that as an org.w3c.dom.Node or Document, or even String. Like:

String expr =
     " declare variable $n external; "
   + " let $count := /root/@count; "
   + " return ( "
   + "   insert node $n into /root, "
   + "   replace value of node $count with $count + 1 "
   + " ) ";
XQConnection xqc = ...;
XQPreparedExpression xqp = xqc.prepareExpression(expr);
// org.w3c.dom.Node node is <node id='{ $count }'/>
xqp.bindNode(new QName("n"), node, null);
xqp.executeQuery();

However, this just leaves me the text { $count } in the attribute. Binding the node as an xs:string value has the same effect.

Of course, this is a nice protection against "XQuery injection". Still then: is there any way to make XQuery Update process an enclosed expression I have in the variables themselves?

(Any other smart ideas to use auto-increment values in XQuery are very welcome too, but then see Auto increment with XQuery Update?)

回答1:

Speaking of injection...why not just pass the node as a string and use basex:eval()?

String node = "<node id='{ $count }'/>";
String expr =
   ...
   + "   insert node xquery:eval($n) into /root, "
   ...

Above, xquery: refers to a BaseX module.

回答2:

The transform expression of XQuery Update may help you here. It can be used to modify existing nodes in main memory. Your example could be rewritten as follows:

declare variable $n external;

let $count := /root/@count
let $n :=
  copy $c := $n
  modify replace value of node $c/node/@id with $count
  return $c
return (
  insert node $n into /root,
  replace value of node $count with $count + 1
)

The external node $n is copied to the variable $c, and the @id attribute of the root node is replaced with the current value of $count.

Of course there are many variants to this approach. You could e.g. insert a new @id attribute instead of replacing the dummy..

  copy $c := $n
  modify insert node attribute id { $count } into $c/node
  return $c

The current syntax of the transform expression is something one needs to get used to first. A better readable syntax may be subject to future versions of the official spec.