Recursive transformation with XSLT: Table to tree

Question

I want to transfor hierarchical elements that represent a tree. The data is provided inside a table in this format:

<?xml version="1.0" encoding="utf-16"?>
<asx:abap xmlns:asx="http://www.sap.com/abapxml" version="1.0">
    <asx:values>
        <CONCEPTS>
            <item>
                <NO>1</NO>
                <NS_PREFIX>de-gaap-ci</NS_PREFIX>
                <XBRL_ID>bs.ass.monetary</XBRL_ID>
                <IS_TUPLE>false</IS_TUPLE>
                <FATHER_ID/>
                <CONTEXT_REF>D-2012</CONTEXT_REF>
                <UNIT_REF>EUR</UNIT_REF>
                <DECIMALS>2</DECIMALS>
                <PRECISION/>
                <IS_NIL>false</IS_NIL>
                <VALUE>12923.00-</VALUE>
            </item>
            <item>
                <NO>2</NO>
                <NS_PREFIX>de-gaap-ci</NS_PREFIX>
                <XBRL_ID>bs.ass.string</XBRL_ID>
                <IS_TUPLE>false</IS_TUPLE>
                <FATHER_ID/>
                <CONTEXT_REF>D-2012</CONTEXT_REF>
                <UNIT_REF/>
                <DECIMALS/>
                <PRECISION/>
                <IS_NIL>false</IS_NIL>
                <VALUE>Test String</VALUE>
            </item>
            <item>
                <NO>3</NO>
                <NS_PREFIX>de-gaap-ci</NS_PREFIX>
                <XBRL_ID>bs.ass.nil</XBRL_ID>
                <IS_TUPLE>false</IS_TUPLE>
                <FATHER_ID/>
                <CONTEXT_REF>D-2012</CONTEXT_REF>
                <UNIT_REF/>
                <DECIMALS/>
                <PRECISION/>
                <IS_NIL>true</IS_NIL>
                <VALUE/>
            </item>
            <item>
                <NO>4</NO>
                <NS_PREFIX>de-gaap-ci</NS_PREFIX>
                <XBRL_ID>bs.ass.tuple1</XBRL_ID>
                <IS_TUPLE>true</IS_TUPLE>
                <FATHER_ID/>
                <CONTEXT_REF>D-2012</CONTEXT_REF>
                <UNIT_REF/>
                <DECIMALS/>
                <PRECISION/>
                <IS_NIL>false</IS_NIL>
                <VALUE/>
            </item>
            <item>
                <NO>5</NO>
                <NS_PREFIX>de-gaap-ci</NS_PREFIX>
                <XBRL_ID>bs.ass.a</XBRL_ID>
                <IS_TUPLE>false</IS_TUPLE>
                <FATHER_ID>bs.ass.tuple1</FATHER_ID>
                <CONTEXT_REF>D-2012</CONTEXT_REF>
                <UNIT_REF/>
                <DECIMALS/>
                <PRECISION/>
                <IS_NIL>false</IS_NIL>
                <VALUE>Value for bs.ass.a</VALUE>
            </item>
            <item>
                <NO>6</NO>
                <NS_PREFIX>de-gaap-ci</NS_PREFIX>
                <XBRL_ID>bs.ass.b</XBRL_ID>
                <IS_TUPLE>false</IS_TUPLE>
                <FATHER_ID>bs.ass.tuple1</FATHER_ID>
                <CONTEXT_REF>D-2012</CONTEXT_REF>
                <UNIT_REF/>
                <DECIMALS/>
                <PRECISION/>
                <IS_NIL>false</IS_NIL>
                <VALUE>Value for bs.ass.b</VALUE>
            </item>
        </CONCEPTS>
        <CONTEXTS>
            <item>
                <ID>D-2012</ID>
                <SCHEME>http://www.rzf-nrw.de/Steuernummer</SCHEME>
                <IDENTIFIER>5117050051729</IDENTIFIER>
                <IS_INSTANT>false</IS_INSTANT>
                <DATE_A>2012-08-28</DATE_A>
                <DATE_B>2012-08-30</DATE_B>
            </item>
            <item>
                <ID>I-2012</ID>
                <SCHEME>http://www.rzf-nrw.de/Steuernummer</SCHEME>
                <IDENTIFIER>5117050051729</IDENTIFIER>
                <IS_INSTANT>true</IS_INSTANT>
                <DATE_A>2012-08-28</DATE_A>
                <DATE_B/>
            </item>
        </CONTEXTS>
        <UNITS>
            <item>
                <ID>EUR</ID>
                <MEASURE_NS>iso4217</MEASURE_NS>
                <MEASURE_VALUE>EUR</MEASURE_VALUE>
            </item>
                        (...)
        </UNITS>
        <NAMESPACES>
            <item>
                <PREFIX>de-gcd</PREFIX>
                <URI>http://www.xbrl.de/taxonomies/de-gcd-2011-09-14</URI>
                <IS_DEFAULT>false</IS_DEFAULT>
            </item>
                        (...)
        </NAMESPACES>
        <SCHEMAS>
            <item>
                <SCHEMA_REF>http://www.xbrl.de/taxonomies/de-gcd-2011-09-14.xsd</SCHEMA_REF>
            </item>
                        (...)
        </SCHEMAS>
    </asx:values>
</asx:abap>

The column IS_TUPLE says that this element is a father node inside the tree. All -elements that have the XBRL_ID of this father inside the column FATHER_ID are supposed to be the children of this father. Hence, if the column FATHER_ID is empty the node has no father.

I want to process these elements recursively with XSLT. The target format is XBRL which is basically XML. Here is my attempt:

<xsl:template match="/">
<xbrl ...>
   <xsl:apply-templates select="/CONCEPTS/item">
      <xsl:with-param name="rec_father_node" select="/" />
   </xsl:apply-templates>
</xbrl>
</xsl:template>

<xsl:template name="tpl_concept" match="/CONCEPTS/item">

    <xsl:param name="rec_father_node"/>

    <xsl:variable name="rec_father_id" select="string($rec_father_node/XBRL_ID/text())"/>

    <xsl:variable name="father_id" select="string(*[local-name()='FATHER_ID'])"/>
    <xsl:variable name="is_tuple" select="string(*[local-name()='IS_TUPLE'])"/>     
    <xsl:variable name="ns_prefix" select="string(*[local-name()='NS_PREFIX'])"/>
    <xsl:variable name="xbrl_id" select="string(*[local-name()='XBRL_ID'])"/>
    <xsl:variable name="name" select="$xbrl_id"/>

    <xsl:if test="$is_tuple = 'false' and $rec_father_id = $father_id">

        <xsl:element name="{$name}">
                      (...)
        </xsl:element>
    </xsl:if>

    <xsl:if test="$is_tuple = 'true' and $rec_father_id = $father_id">
        <xsl:element name="{$name}">
            <xsl:choose>
                <xsl:when test="$is_nil = 'true'">
                    <xsl:attribute name="xsi:nil">true</xsl:attribute>
                </xsl:when>
                <xsl:otherwise>
                    <xsl:apply-templates select="/CONCEPTS/item">
                        <xsl:with-param name="rec_father_id" select="current()" />
                    </xsl:apply-templates>                  
                </xsl:otherwise>
            </xsl:choose>
        </xsl:element>
    </xsl:if>       
</xsl:template>

My idea was the following: nodes that are no father have the string value of '' inside the column FATHER_ID. Therefore, I pass the root node to the parameter rec_father_node to get this initial string. With the recursive call of the same template the parameter rec_father_node contains the node of this father so I can read out the XBRL_ID of this father to only add elements that have this value inside the column FATHER_ID.

My problem is the following: the parameter rec_father_node looses its value passed by the recursion call. The value is always the root node which is passed by the initial <xsl:apply-templates .../> call.

Is there a way to pass a parameter to recursive template calls? Or in general, how can I transform this flat table into a XML node tree with nested elements?

The target of the transformation should look like this (taken names from the example above and added some other nodes for clarity):

<?xml version="1.0" encoding="UTF-8"?>
<xbrl>
    <bs.ass.monetary>12923.00-</bs.ass.monetary>
    <bs.ass.string>Test String</bs.ass.string>
    <bs.ass.nil></bs.ass.nil>
    <bs.ass.tuple1>
        <bs.ass.a>Value for bs.ass.a</bs.ass.a>
        <bs.ass.b>Value for bs.ass.b</bs.ass.b>
    </bs.ass.tuple1>
</xbrl>

Thanks for any help or comments!

Answer 1

What you could do is firstly define a key to look up 'child' items based on their FATHER_ID elements.

<xsl:key name="children" match="item" use="FATHER_ID" />

Firstly though you would start off by matching items with no father

<xsl:apply-templates select="item[FATHER_ID='']"/>

Then, for items which are 'tuples' you could match them, and recursively match their children with a key

<xsl:template match="item[IS_TUPLE='true']">
   <xsl:element name="{XBRL_ID}">
      <xsl:apply-templates select="key('children', XBRL_ID)" />
   </xsl:element>
</xsl:template>

You would also have another template for item elements, which would match non-tuples, which would output whatever value you like.

Try the following XSLT

<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
   <xsl:output method="xml" indent="yes"/>
   <xsl:key name="children" match="item" use="FATHER_ID"/>

   <xsl:template match="/">
         <xsl:apply-templates select="//CONCEPTS"/>
   </xsl:template>

   <xsl:template match="CONCEPTS">
      <xbrl>
         <xsl:apply-templates select="item[FATHER_ID='']"/>
      </xbrl>
   </xsl:template>

   <xsl:template match="item[IS_TUPLE='true']">
      <xsl:element name="{XBRL_ID}">
         <xsl:apply-templates select="key('children', XBRL_ID)"/>
      </xsl:element>
   </xsl:template>

   <xsl:template match="item">
      <xsl:element name="{XBRL_ID}">
         <xsl:value-of select="VALUE" />
      </xsl:element>
   </xsl:template>
</xsl:stylesheet>

When applied to your sample, you get the following

<xbrl>
   <bs.ass.monetary>12923.00-</bs.ass.monetary>
   <bs.ass.string>Test String</bs.ass.string>
   <bs.ass.nil></bs.ass.nil>
   <bs.ass.tuple1>
      <bs.ass.a>Value for bs.ass.a</bs.ass.a>
      <bs.ass.b>Value for bs.ass.b</bs.ass.b>
   </bs.ass.tuple1>
</xbrl>