Insert XML node at a specific position of an existing document

Question

Hi,

I have an existing XML document with some optional nodes and I want to insert a new node, but at a certain position.

The document looks something like this:

<root>
  <a>...</a>
  ...
  <r>...</r>
  <t>...</t>
  ...
  <z>...</z>
</root>

The new node (<s>...</s>) should be inserted between node <r> and <t>, resulting in:

<root>
  <a>...</a>
  ...
  <r>...</r>
  <s>new node</s>
  <t>...</t>
  ...
  <z>...</z>
</root>

The problem is, that the existing nodes are optional. Therefore, I can't use XPath to find node <r> and insert the new node after it.

I would like to avoid the "brute force method": Search from <r> up to <a> to find a node that exists.

I also want to preserve the order, since the XML document has to conform to a XML schema.

XSLT as well as normal XML libraries can be used, but since I'm only using Saxon-B, schema aware XSLT processing is not an option.

Does anyone have an idea on how to insert such a node?

thx, MyKey_

Answer 1

You must use a brute force search since you have no static path to find the insert location. My approach would be to use a SAX parser and read the document. All nodes are copied to the output unmodified.

You'll need a flag sWasWritten which is why you can't use a normal XSLT tool; you need one where you can modify variables.

As soon as I see a node > r (t, u, ..., z) or the end-tag of the root node, I'd write the s node unless sWasWritten was true and set the flag sWasWritten.

Answer 2

The XML Spec specifies that elements are not in a specific (sorted) order. By using an XSD you can set rules about which elements need to be where in the XML tree in which amount, but not in which order.

<set>
  <a /><b />
</set>

or

<set>
  <b /><a />
</set>

result in the same XML DOM.

The order of elements should not matter since XML is meant for data transferring, not sorting information (for that you can use things like XSL). Maybe you can search the answer in an easier approach by letting go of the order in which the elements are placed in your XML. (or go with the answer of Aaron Digulla by sax parsing, mtfbwy)

Answer 3

[Replaced my last answer. Now I understand better what you need.]

Here's an XSLT 2.0 solution:

<xsl:stylesheet version="2.0"
  xmlns:xsl="http://www.w3.org/1999/XSL/Transform"&gt;

  <xsl:template match="/root">
    <xsl:variable name="elements-after" select="t|u|v|w|x|y|z"/>
    <xsl:copy>
      <xsl:copy-of select="* except $elements-after"/>
      <s>new node</s>
      <xsl:copy-of select="$elements-after"/>
    </xsl:copy>
  </xsl:template>

</xsl:stylesheet>

You have to explicitly list either the elements that come after or the elements that come before. (You don't have to list both.) I would tend to choose the shorter of the two lists (hence "t" - "z" in the above example, instead of "a" - "r").

OPTIONAL ENHANCEMENT:

This gets the job done, but now you need to maintain the list of element names in two different places (in the XSLT and in the schema). If it changes much, then they might get out of sync. If you add a new element to the schema but forget to add it to the XSLT, then it won't get copied through. If you're worried about this, you can implement your own sort of schema awareness. Let's say your schema looks like this:

<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"&gt;

  <xs:element name="root">
    <xs:complexType>
      <xs:sequence>
        <xs:element name="a" type="xs:string"/>
        <xs:element name="r" type="xs:string"/>
        <xs:element name="s" type="xs:string"/>
        <xs:element name="t" type="xs:string"/>
        <xs:element name="z" type="xs:string"/>
      </xs:sequence>
    </xs:complexType>
  </xs:element>

</xs:schema>

Now all you need to do is change your definition of the $elements-after variable:

  <xsl:variable name="elements-after" as="element()*">
    <xsl:variable name="root-decl" select="document('root.xsd')/*/xs:element[@name eq 'root']"/>
    <xsl:variable name="child-decls" select="$root-decl/xs:complexType/xs:sequence/xs:element"/>
    <xsl:variable name="decls-after" select="$child-decls[preceding-sibling::xs:element[@name eq 's']]"/>
    <xsl:sequence select="*[local-name() = $decls-after/@name]"/>
  </xsl:variable>

This is obviously more complicated, but now you don't have to list any elements (other than "s") in your code. The script's behavior will automatically update whenever you change the schema (in particular, if you were to add new elements). Whether this is overkill or not depends on your project. I offer it simply as an optional add-on. :-)

Answer 4

An XPath solution:

/root/(.|a|r)[position()=last()]

You must explicitly include all the nodes up to the one you want, so that you'll need a different XPath expression for each node you want to insert after. For example, to place it immediately after <t> (if it exists):

/root/(.|a|r|t)[position()=last()]

Note the special case of when none of the preceding nodes are present: it returns <root> (the "."). You'll need to check for this, and insert the new node as the first child of root, instead of after it (the usual case). This isn't so bad: you'd have to handle this special case in some way, anyway. Another way to handle this special case is the following, which returns 0 nodes if there are no preceding nodes.

/root/(.|a|r|t)[position()=last() and position()!=1]

Challenge: can you find a better way to handle this special case?

Answer 5

Thanks for the explanations. Could anybody tell me how to insert some XML between two grand child nodes. For example I want to insert ... between ... and ... in this file ... ... ... ...