Insert XML node at a specific position of an existing document

Question

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_

Solution

[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">

  <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">

  <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. :-)

Comments

[Evan Lenz]

SAX processing will work as you suggest. But XSLT is quite capable for the task as well (see my answer).

[Aaron Digulla]

This doesn't work when there is no 'r' node (as per the original question: All nodes are optional). How would the template look when you can't rely on any node to exist?

[Evan Lenz]

Oops, you're right. I had mis-read the original post. Now I've completely replaced the answer. Thanks.

[13ren]

That's really cool. Slight refinement: in deriving $elments-after, use a variable instead of 's', so you can automatically handle inserting after any child of <root>.

[Evan Lenz]

Agreed. The "s" is pretty hidden among all those implementation details.

[MyKey_]

Wow, nice solution. I would never have thought of parsing the schema. Thanks a lot.

[London]

@Evan Lenz is it possible to do this somehow from java using this schema?