Merging data from an XML node to another node using XSLT

Question

Using XSLT, I am trying to figure out how to merge/update the data in a set of nodes with data from another set of nodes. The nodes have the same schema, but different parents. The data needs to be merged based on a shared parent attribute. In the example below, data is being copied from Principal to Driver. Can anyone help me out here?

Input File:

<Info>
  <Principal id="Insured">
    <PersonName>
      <GivenName>Jane</GivenName>
      <OtherGivenName>A</OtherGivenName>
      <Surname>Doe</Surname>
    </PersonName>
    <PersonInfo>
      <BirthDate>01-01-1980</BirthDate>
      <MaritalStatus>M</MaritalStatus>
    </PersonInfo>
    <PrincipalInfo></PrincipalInfo>
  </Principal>
  <Policy>
    <Driver id="Insured">
      <PersonName>
        <GivenName>Jane</GivenName>
        <Surname>Smith</Surname>
      </PersonName>
      <PersonInfo>
        <BirthDate>01-01-1980</BirthDate>
        <MaritalStatus>S</MaritalStatus>
        <Occupation>Manager</Occupation>
      </PersonInfo>
    </Driver>
    <PolicyInfo></PolicyInfo>
  </Policy>
</Info>

Desired Result:

<Info>
  <Principal id="Insured">
    <PersonName>
      <GivenName>Jane</GivenName>
      <OtherGivenName>A</OtherGivenName>
      <Surname>Doe</Surname>
    </PersonName>
    <PersonInfo>
      <BirthDate>01-01-1980</BirthDate>
      <MaritalStatus>M</MaritalStatus>
    </PersonInfo>
    <PrincipalInfo></PrincipalInfo>
  </Principal>
  <Policy>
    <Driver id="Insured">
      <PersonName>
        <GivenName>Jane</GivenName>
        <OtherGivenName>A</OtherGivenName>
        <Surname>Doe</Surname>
      </PersonName>
      <PersonInfo>
        <BirthDate>01-01-1980</BirthDate>
        <MaritalStatus>M</MaritalStatus>
        <Occupation>Manager</Occupation>
      </PersonInfo>
    </Driver>
    <PolicyInfo></PolicyInfo>
  </Policy>
</Info>

Is it a real deal? You basically replace PersonName with new values. No merging occurs. — Flack
– Flack, Commented Feb 12, 2011 at 16:42
There may be extra nodes in the Driver subnodes, that I want to leave, so just replacing the entire node doesn't work. Note the <Occupation> in my example. — Tim B
– Tim B, Commented Feb 12, 2011 at 17:02
Also, the node from <Principal> may not yet exist in the <Driver>, note the <OtherGivenName>. — Tim B
– Tim B, Commented Feb 12, 2011 at 17:12
Good question, +1. See my answer for a complete, short and easy solution that uses the most fundamental and powerful XSLT design pattern: the overriding of the identity rule. A thorough explanation is provided. — Dimitre Novatchev
– Dimitre Novatchev, Commented Feb 12, 2011 at 18:26

Dimitre Novatchev · Accepted Answer · 2011-02-12 18:57:34Z

Here is a complete solution:

<xsl:stylesheet version="1.0"
 xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
 <xsl:output omit-xml-declaration="yes" indent="yes"/>
 <xsl:strip-space elements="*"/>

 <xsl:key name="kPrincipalById" match="Principal"
  use="@id"/>

 <xsl:key name="kPrincipalChild" match="Principal/*/*"
  use="concat(../../@id, name())"/>

 <xsl:template match="node()|@*">
  <xsl:copy>
   <xsl:apply-templates select="node()|@*"/>
  </xsl:copy>
 </xsl:template>

 <xsl:template match="Driver/*">
  <xsl:variable name="vPrincipal"
   select="key('kPrincipalById', ../@id)"/>

   <xsl:copy>
    <xsl:apply-templates select="@*"/>
    <xsl:apply-templates select=
     "$vPrincipal/*[name()=name(current())]/*"/>
    <xsl:apply-templates select=
     "*[not(key('kPrincipalChild',
                 concat(../../@id,name())
                 )
            )
        ]"/>
   </xsl:copy>
 </xsl:template>
</xsl:stylesheet>

when this transformation is applied on the provided XML document:

<Info>
    <Principal id="Insured">
        <PersonName>
            <GivenName>Jane</GivenName>
            <OtherGivenName>A</OtherGivenName>
            <Surname>Doe</Surname>
        </PersonName>
        <PersonInfo>
            <BirthDate>01-01-1980</BirthDate>
            <MaritalStatus>M</MaritalStatus>
        </PersonInfo>
        <PrincipalInfo></PrincipalInfo>
    </Principal>
    <Policy>
        <Driver id="Insured">
            <PersonName>
                <GivenName>Jane</GivenName>
                <Surname>Smith</Surname>
            </PersonName>
            <PersonInfo>
                <BirthDate>01-01-1980</BirthDate>
                <MaritalStatus>S</MaritalStatus>
                <Occupation>Manager</Occupation>
            </PersonInfo>
        </Driver>
        <PolicyInfo></PolicyInfo>
    </Policy>
</Info>

the wanted, correct result is produced:

<Info>
   <Principal id="Insured">
      <PersonName>
         <GivenName>Jane</GivenName>
         <OtherGivenName>A</OtherGivenName>
         <Surname>Doe</Surname>
      </PersonName>
      <PersonInfo>
         <BirthDate>01-01-1980</BirthDate>
         <MaritalStatus>M</MaritalStatus>
      </PersonInfo>
      <PrincipalInfo/>
   </Principal>
   <Policy>
      <Driver id="Insured">
         <PersonName>
            <GivenName>Jane</GivenName>
            <OtherGivenName>A</OtherGivenName>
            <Surname>Doe</Surname>
         </PersonName>
         <PersonInfo>
            <BirthDate>01-01-1980</BirthDate>
            <MaritalStatus>M</MaritalStatus>
            <Occupation>Manager</Occupation>
         </PersonInfo>
      </Driver>
      <PolicyInfo/>
   </Policy>
</Info>

Explanation:

The identity rule/template copies every node "as-is". The use and overriding of the identity rule is the most fundamental and powerful XSLT design pattern.
There is just one additional template that overrides the identity rule for children-elements of Driver. It copies (and effectively replaces the same-named grand-child elements of Driver with the corresponding) grand-child elements of Principal. Then it still processes (copies) those grand-children elements of Driver that do not have corresponding grand-children elements of Principal
For convenient access to Principal and its grand-children -- by id and id++name(), there are two keys defined and used.

I'm new to XSLT and still don't completely understand your solution, but it appears to work for every test I've thrown at it.
@Tim-B: You are welcome. To better understand some key points in this solution, please, read more on the subject of "identity rule" and xslt keys. I have edited my answer and now the explanation has links to these two topics. To learn more about XSLT/XPath, you can use the resources identified in this answer: stackoverflow.com/questions/339930/…

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