Merging two XML source documents with XSLT

Question

XML file one could have 1000 - 6000 forms; XML file two could have one to 100 or more. I want to replace any identical form in file one with file two. If it exists in file 2 but not in file one I want to add it to file 1. After the files are merged I want to run it against my XSLT. I am using a 2.0 stylesheet and a Saxon parser.

File 1:

<Forms>
<Form name="fred" date="10/01/2008"/>
<Form name="barney" date="12/31/2009"/>
<Form name="wilma" date="12/31/2010"/>
</Forms>

File 2:

<Forms>
<Form name="barney" date="01/31/2010"/>
<Form name="betty" date="6/31/2009"/>
</Forms>

Merged file should look like:

<Forms>
<Form name="fred" date="10/01/2008"/>
<Form name="barney" date="01/31/2010"/>
<Form name="wilma" date="12/31/2010"/>
<Form name="betty" date="6/31/2009"/>
</Forms>

Answer 1

If maintaining document order is not a priority:

<xsl:variable name="forms1" select="document('forms1.xml')/Forms/Form" />
<xsl:variable name="forms2" select="document('forms2.xml')/Forms/Form" />

<xsl:variable name="merged" select="
  $forms1[not(@name = $forms2/@name)] | $forms2
" />

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

<xsl:template match="Form">
  <!-- for the sake of the example; you can use a more specialized template -->
  <xsl:copy-of select="." />
</xsl:template>

If maintaining document order is a priority for whatever reason…

<xsl:template match="/">
  <!-- check values of file 1 sequentially, and replace them if needed -->
  <xsl:for-each select="$forms1">
    <xsl:variable name="this"  select="." />
    <xsl:variable name="newer" select="$forms2[@name = $this/@name]" />
    <xsl:choose>
      <xsl:when test="$newer">
        <xsl:apply-templates select="$newer" />
      </xsl:when>
      <xsl:otherwise>
        <xsl:apply-templates select="$this" />
      </xsl:otherwise>
    </xsl:choose>
  </xsl:for-each>
  <!-- append any values from file 2 that are not in file 1 -->
  <xsl:apply-templates select="$forms2[not(@name = $forms1/@name)]" />
</xsl:template>